The hosts took the lead 10 minutes into the second half through Jon Taylor, but Darren Fletcher's scorching strike with just under 20 minutes remaining levelled things up.
The game went all the way to penalties, and it was Lee Angol who saw his decisive spot kick saved as the Baggies set up a meeting with Reading in the last 16.
Find out how all of the action unfolded courtesy of Sports Mole's live minute-by-minute coverage below.
6.45pmGood evening! Thank you very much for joining Sports Mole for tonight's FA Cup replay between Peterborough United and West Bromwich Albion at London Road. There is one last place in the fifth round up for grabs following West Ham's dramatic victory over Liverpool in the other replay last night, and Peterborough could end the fourth round with a big shock should they claim that spot tonight. Let's start with a look at the two teams...
6.47pmPETERBOROUGH SUBS: Henry, Ntlhe, Oztumer, Maddison, Da Silva-Lopes, Coulibaly, Toffolo
6.49pmWhile we wait for West Brom to grace us with their team news, let's take a quick look at what we can expect from the side named by the hosts this evening. Their key man and main goal threat is likely to be Angol, who leads the club's goalscoring charts in League One this season with seven from 22 appearances. The 21-year-old is not a striker, however, so the Baggies will need to be wary of keeping track of him this evening.
6.51pmMany of Peterborough's goals this season have come from midfield or even further back, with Angol, Oztumer, Taylor, Anderson and Bostwick combining for 25 between them. There is no Anderson tonight, but Taylor and Bostwick are both handed starts, while Oztumer drops to the bench.
6.53pmIn all, Graham Westley has made five changes to the side that beat Chesterfield at the weekend, although three of those are enforced with Smith and Ricardo suspended and Nichols ineligible. Zakuani is fit again following a recent hamstring injury, and having come off the bench at the weekend he is handed a start tonight, which is a big boost for the home side here.
6.55pmCoulthirst, who got the first equaliser against West Brom in the original tie between these two sides, and Taylor, who rescued the draw with a late equaliser of his own, both start once again tonight. The men to come into the side from the weekend match are Zakuani, Baldwin, Beautyman, Fox and Samuelsen.
6.59pmWest Brom, meanwhile, are suffering from something of a defensive crisis at the moment, with a lot of players currently sidelined through injury. Evans, Dawson and Brunt are all deemed not fit enough to play any part in tonight's game, and there must have been some patching up along the way too as both Olsson and McAuley, who do start, were considered doubts due to hamstring and thigh injuries respectively.
7.01pmPulis has managed to get by without naming too much of a makeshift back four, then, with only one change from the side that lost to Newcastle at the weekend when it comes to the defence. That sees Gamboa replace Dawson, while behind them Foster continues to tend to goal - and he might be in for a busy night given Peterborough's scoring record this season.
7.03pmIt will be interesting to see whether Pulis goes for a 4-4-2 formation this evening or his usual system, with Berahino or perhaps Rondon dropping into a slightly deeper role. Fletcher was a little more advanced than usual at the weekend, with Sandro playing alongside Yacob in a holding midfield role, but he will probably be the one to drop a little deeper should Pulis stick with a 4-2-3-1 formation tonight.
7.05pmThe biggest inclusion for the Baggies is that of Berahino, who cannot buy a start in the Premier League at the moment despite West Brom's troubles in front of goal. He and Rondon sat out at the weekend as Anichebe was given the nod up front, but both play tonight. Berahino's FA Cup record really is impressive for West Brom too - he has scored eight goals and assisted four more in his last six outings in this competition.
7.07pmIt will be Berahino that the hosts are most wary of this evening, then, but despite the gap in the football pyramid between these two sides, Graham Westley will feel that his side have a real chance to pull off an upset here. They performed well enough in the original tie to force this replay, showing good character to twice come from behind, and with London Road cheering them on tonight, it could be a special night for Peterborough.
7.09pmThey don't come into the match in the best of form having only won one of their last five games in all competitions, but crucially that victory did come in their most recent outing to end a four-game winless streak. Their victory on Saturday was their first since the FA Cup third round triumph over Preston North End, and their first in the league since a 3-2 success over Sheffield United on January 2.
7.11pmDespite that, they remain very much in the playoff hunt in League One as things stand. They currently sit eighth in the third tier and, in terms of points, are actually closer to the relegation zone than they are to leaders Burton, but only three points separate them from Millwall in sixth. That is by no means an unassailable gap for the Posh, particularly when they boast a game in hand over some of the teams around them in the table.
7.13pmTheir success in the league so far has been based on a potent attacking threat, with Peterborough having found the back of the net 54 times in their 29 League One outings this season, a tally only Gillingham can beat. The goals have just dried up a little in recent weeks, though, with Peterborough having only scored twice in their last four league games.
7.15pmAs if often the case with sides who score a lot of goals but don't find themselves in the title race, the defensive record is not as impressive. They have conceded 41 times in their 29 games, which is the most in the top nine of League One, while only Barnsley have conceded more in the top 14 of the table. They do boast a decent goal difference compared to the teams immediately around them in the table, but it is fair to say that it isn't a perfectly balanced side.
7.17pmThe Posh have lost two of their last four games in all competitions following a run of just two losses in 13 before that, but at home they have only been beaten once in their last nine outings. That defeat did come in their most recent home game, but it was also against League One leaders Burton Albion. That ended an unbeaten streak that included six wins and two draws since their 3-2 loss at the hands of Wigan back in October.
7.19pmBurton are actually one of the teams that Peterborough have beaten in the FA Cup this season, overcoming their League One rivals in the first round of the competition before later ousting Luton and Preston, all without conceding a goal. They are one of only two League One sides remain in the competition this season, along with Shrewsbury Town, and victory tonight would see them qualify for the fifth round for the first time since 1986.
7.21pmThe odds will be stacked in favour of the visitors, though, despite West Brom arriving at London Road in questionable form. The Baggies are by no means out of the mire when it comes to the Premier League relegation battle this season, although they are still closer to the top half than they are to the bottom three. Still, the six-point gap that separates them from the relegation zone could quickly disappear if they don't improve soon.
7.23pmWest Brom are winless in their last five Premier League outings since a 2-1 triumph over Stoke City at the start of January, and Pulis himself warned his side that they are fighting for their Premier League lives following their defeat to Newcastle at the weekend. Peterborough will be hoping that their league position means that they won't be giving this evening their full attention, although is would be a surprise if Pulis hasn't got his side up for tonight's game.
7.25pmThat defeat to Newcastle was describe by Tony Pulis as the "most disappointing performance" since he arrived at the club, with Aleksandar Mitrovic's first-half goal proving to be the difference at St James' Park. The biggest problem currently lies in the final third, with West Brom having failed to even have a shot on target in three of their last four league outings. Only Aston Villa have scored fewer goals than the Baggies in the top flight all season, which makes it all the more interesting why Pulis has not yet turned to Berahino as a regular starter.
7.27pmNot only do West Brom not score enough goals, but they don't create enough chances either - or at least their finishing is not up to scratch, despite the big-money arrival of Rondon in the summer. They have had a league-low 76 shots on target from their 25 games this season, a pitiful average of just over three per game, while their possession average of 39.8% is also the lowest figure in the Premier League.
7.29pmA 1-0 victory over Bristol City in the third round of this competition is West Brom's only away victory in their last eight matches in all competitions, and one of only two on the road in their last 11, a run that stretches back to September. They have lost six of those 11 away games since beating lowly Aston Villa in the second month of the campaign.
7.31pmIn the FA Cup, though, they have performed fairly well over the last couple of seasons. Their only defeat in their last seven matches in this competition came in the quarter-finals last term, when they were beaten by eventual finalists Aston Villa. That is also the only game of those seven in which they have failed to score, finding the back of the net 18 times in the other six.
7.33pmThey have, however, needed replays in both rounds so far this season. Bristol City were their opponents in the third round and, after staying in the competition by the skin of their teeth courtesy of James Morrison's 95th-minute equaliser at The Hawthorns, they went on to win the replay 1-0 at Ashton Gate. Victory tonight to back that one up with give them consecutive away FA Cup wins for the first time since 2008.
7.35pmPREDICTION: We're 10 minutes from kickoff at London Road, which means that it is time for a prediction! West Brom are clearly the favourites tonight, being from two divisions above Peterborough, but the hosts have been in good goalscoring form this season and the Baggies can barely get a shot on target at the moment. This is a potential banana skin for Pulis's side, and I can see them slipping up. I'll go for a 2-1 home win.
7.37pmThis year is the third in the last eight that these two sides have been drawn together in the FA Cup, having never faced off in this competition before 2008. Peterborough have not won any of the previous ties, losing at London Road on each occasion. In 2008, West Brom won 3-0 here, while a year later it was 2-0 after Peterborough had forced a relay with a 1-1 draw at The Hawthorns.
7.39pmIndeed, in all competitions West Brom have won each of their last four away games against Peterborough, which includes a 3-2 victory here on their last visit - a Championship clash in August 2009. Luke Moore scored twice and Chris Brunt once to send the Baggies into a three-goal lead, before they held off a late comeback to seal the three points.
7.41pmThey looked to be on course for another victory in the original tie at The Hawthorns last month thanks to Berahino's 14th-minute strike, but a remarkable final 11 minutes saw Peterborough rescue a draw from the game. Coulthirst levelled things up for the Posh in the 79th minute, only for Berahino to restore the Baggies' lead five minutes later. This time that lead would last only two minutes, however, as Taylor equalised again to clinch a 2-2 draw.
7.43pmWest Brom have not been knocked out of the FA Cup by a team from outside the Premier League since 2011, and should they extend that run this evening then it is their conquerors on that occasion - Reading - who await them in the fifth round.Last night's replay was an action-packed classic that provided drama all the way until the 120th minute and beyond, and we could be in for another memorable evening here.
1 minKICKOFF: West Brom get us underway at London Road as they look to avoid an upset and book their place in the last 16 of the FA Cup.
3 minWest Brom have dominated the possession early in this match. They are just knocking the ball about, going nowhere fast but doing a good job of settling into the game and quietening down the crowd.
5 minPeterborough win an early corner, which West Brom struggle to deal with, but the ball bounces safely for them as a blue shirt in the middle can't react quickly enough. Fox goes for goal on the follow-up from a tight angle, but his dragged effort hits his own man and goes behind for a goal kick.
7 minCHANCE! Magnificent piece of defending from McClean as Peterborough begin to apply a bit of pressure. Samuelsen dances his way into the box before squaring the ball to Beautyman, who is in space and looks certain to score. However. McClean flies in from nowhere to make a brilliant block, while moments later and effort from Taylor is gather by Foster.
9 minMcClean dives in to a heavy challenge on Samuelsen, which draws angry shouts from the crowd and urges to send the West Brom man off. Referee Mike Jones deals with it well, though, and only awards a free kick. Even that may have been a bit harsh - it was a 50-50 and McClean won it.
11 minSHOT! Taylor isn't afraid to have a go from range tonight. He wins the ball inside the West Brom half and brings it forward before letting fly, but his drags his strike a little and it ends up comfortably wide of the target.
13 minSAVE! It is the League One hosts who are asking all of the questions right now. They take a free kick early here, with the ball being played short to Fox, who wasted no time in going for goal from range. It is a dipping striker, and Foster is needed to just shovel it behind.
14 minThe resulting delivery is met by Bostwick, who worked really hard to get across his man at the front post there. Foster is needed to make the save again, although the referee's whistle has already gone for a free kick to West Brom.
15 minWest Brom make an increasingly rare foray into Peterborough temperature and win a corner, but that comes to nothing and the hosts immediately look to break. Coulthirst uses his pace and beats one man when cutting inside, but a heavy touch allows a challenge to be made.
16 minWell, he was a major doubt for this match, and any gamble that Pulis decided to make on the fitness of McAuley has backfired here. The defender pulled something when chasing Coulthirst back a moment ago, and he has now been forced to limp off.
18 minMuch to the annoyance of Pulis, Pocognoli is taking an age to get his kit on on the sidelines, and West Brom are still having to struggle on with 10 men here.
19 minWEST BROM SUB: Finally the change is made, with Sebastien Pocognoli replacing the injured McAuley.
20 minCHANCE! Big chance for Peterborough to take the lead! A free kick is played short and then fed low towards the edge of the box before being touched off to Beautyman in space inside the box. He allows the ball to run across his body, however, which makes it a more difficult angle, and Foster makes a big save from the eventual effort.
22 minPeterborough have been quiet comfortable the better side in this match so far. The Baggies are struggling to even get out of their own half and have given away too many free kicks in decent crossing positions.
23 minThe latest of those free kicks is swung into a good area by Fox, but it is just too high for Bostwick and West Brom are able to clear.
25 minPeterborough have had 62% possession in this match so far, which means West Brom are also seeing a similar amount of the ball as they do in the Premier League. The visitors are also yet to have a shot on target, so at the moment they are finding things just as difficult as they do in the top flight.
27 minThe Baggies just haven't been able to get anything going so far. Berahino and Rondon have barely featured in the game at all, certainly not as a partnership, and it is all looking quite comfortable for the home side at the moment.
29 minA final warning from Mike Jones here as Yacob drags down Samuelsen, who span away from two players before driving forward with the ball. The fans want a card, but the referee is being lenient.
30 minMore leniency from the referee here as Baldwin scythes down Gamboa on the left flank. It probably deserves a yellow, but Mike Jones is at least consistent and gives him a warning. Free kick for West Brom in a good crossing position...
31 minThe ball is driven in low towards the near post and goes all the way through to Alnwick, who responds just in time despite seeing it late and having bodies racing in front of him.
33 minIt is more even now than it was during the spell when Peterborough were on top, and you wonder whether the hosts will live to rue not making the most of that time in the ascendency. The visitors are beginning to settle a bit.
35 minAn ambitious penalty shout from Taylor as he fails to collect a ball forward and it bounces up and maybe just brushes his arm, but it would have been a very harsh one.
36 minCLOSE! West Brom threaten for the first time tonight, and it is a lovely effort from Berahino. It comes from nothing, with Foster firing a long ball upfield which Rondon knocks down to his strike partner. Berahino tries to steer a side-footed effort towards the top corner from 20 yards, but it goes just past the post. That wasn't far away at all.
37 minReally good work from Taylor down the left as he bursts past a couple of defenders into the final third. He then plays an inviting low ball right across the area, but there are no blue shirts there to knock it in.
39 minAgain Taylor goes on his merry way down the left, but this time he is halted in his tracks. He is one of a few Peterborough players to have impressed so far this evening.
40 minCHANCE! Another opening from West Brom as Gamboa overlaps on the right to collect a through-ball before lifting a cross into the middle. Rondon can't connect at the near post and Alnwick also completely misses it, but Baldwin is there at the back post to make a fine block from McClean's effort.
41 minCHANCE! From the resulting corner the ball drops to Chester in space at the back post, but he scuffs his shot when he really should score from close range, and Fletcher can't get out of the way quickly enough as the ball hits his arm and goes behind.
43 minCHANCE! The visitors are knocking on the door now. Another set piece creates another chance as this time a free kick is swung into the middle, and Olsson climbs highest to meet the ball. However, he just gets too much on it and sends his header over the crossbar.
44 minSHOT! Berahino is the latest to threaten for the Baggies, with the ball being worked back to Berahino on the left side of the area. He tries to open his body and curl one into the far top corner, but it flies just too high.
45 minThere will be a minimum of one minute stoppage time at the end of this first half.
45+1 minHALF TIME: Peterborough United 0-0 West Bromwich Albion
8.40pmThe first half comes to an end at London Road, and we're still goalless between Peterborough and West Brom here. There have been chances at both ends of the field, with Peterborough controlling things for much of the opening half an hour, but the Baggies came back into it towards the end of the half and were knocking on the door in the closing stages.
8.42pmThe hosts almost took the lead inside the opening 10 minutes when Samuelsen danced his way into the box before squaring the ball for Beautyman, who was only denied by a flying block from McClean. Taylor and Fox also threatened with long-range strikes with less than 15 minutes on the clock as Peterborough continued to knock on the door in the opening exchanges.
8.44pmFoster then denied Beautyman after 20 minutes when the midfielder allowed the ball to run across his body inside the box, which made things a little more difficult for him. The angle favoured the West Brom keeper, and sure enough he made a big save at the near post to keep the scores goalless.
8.46pmIt was West Brom who ended the half on top, though, with Berahino threatening for the first time with 10 minutes remaining in the half when he steered a well-controlled effort narrowly past the post. Chester squandered a great chance moments later when the ball dropped to him at the back post, but he scuffed his finish and failed to steer it on target when he really should have scored.
46 minKICKOFF: Peterborough get us back underway for this second half as they look to join Shrewsbury as the only League One sides in the fifth round.
47 minAlmost an early chance in this second half for Peterborough as Taylor races down the left flank and cuts the ball back for Coulthirst. He collects the ball and tries to beat his man in the area, but Chester makes a crucial challenge.
47 minYELLOW CARD! Yacob got his final warning in the first half, and now he goes into the book for a challenge from behind on Coulthirst.
48 minSAVE! Taylor takes the resulting free kick on, but it lacks power and Foster is across in enough time to make a comfortable catch.
50 minThere have been no signs in this second half so far that the pattern of the game is going to change here. Peterborough are still having a really good go and, just as they did in the opening 45 minutes, they have started the brighter here.
51 minCHANCE! Another chance for the home side as Fox dummies his way past one man on the left flank before playing a low pass in to Coulthirst. He collects the ball and looks to curl an effort into the far corner, but it is a couple of yards off target.
53 minAnother sweeping attack from the hosts sees them come down the right this time, but the cross into the middle flies straight into the arms of Foster. The keeper tries to get his side going in the opposite direction quickly, but he is too eager and his throw goes straight out of play.
55 minGOAL! Peterborough 1-0 West Brom (Jon Taylor)
55 minPeterborough get the opening goal! The hosts take the lead 10 minutes into the second half, and it is Taylor, who has looked dangerous all evening, who gets it. Angol collects a wayward pass on the halfway line and immediately spins to charge forward at the West Brom defence before feeding it out to Taylor. He is given too much space on the edge of the area and, while his shot is not cleanly struck, it takes a nick off Chester and beats Foster.
57 minWhat a huge goal that is Peterborough, and it is no more than they deserve. With the exception of a 10-minute spell at the end of the first half they have been quite comfortably the better team so far today, and look well on course for a shock here.
59 minThe deflection on the goal just made it bobble up a little, but Foster may still feel that he should have saved it. He seemed to get there in time, but the ball just squirmed away from his outstretched hand.
60 minSAVE! Angol carries the ball forward and is allowed to keep on coming by the West Brom defence. His eventual shot is disappointing, however, as he steers a rather tame effort into the arms of Foster.
62 minWEST BROM SUB: Time for a change from Pulis as Yacob departs to be replaced by Stephane Sessegnon. It is an attacking switch from the visitors!
64 minThe hosts will be eager for a second goal to give them a cushion and, with the players at their disposal here, the more West Brom push for an equaliser the more Peterborough may fancy their chances of getting a second. They have pace on the break and have shown that they can hurt the visitors already.
66 minInteresting situation here as, during a break in play, Olsson strolls back towards Foster to exchange angry words with his goalkeeper. Things just aren't going well for the Baggies at the moment.
68 minTaylor wants a penalty here as he lets fly from just inside the area and sees his shot blocked. It did appear to hit an arm, but the proximity and position of the defender's hands mean that the referee made the right decision there.
69 minAlmost a second for Peterborough as Bostwick wins a header from a corner and nods it down into a dangerous area. In the end it is Gardner who gets the final touch, putting it over his own crossbar.
70 minSHOT! West Brom may need a wonder goal the way this match is going, and Gardner very nearly provides one of the highest order. The ball arrives to him at an awkward height on the edge of the box from the right flank, but he gets a beauty of a volley away that drifts a couple of yards wide.
71 minGOAL! Peterborough 1-1 West Brom (Darren Fletcher)
71 minAnd there is that wonder goal! Fletcher slams his side level again with a cracking strike from the edge of the box. It was McClean who created the chance down the left, pulling the ball back into the middle. It eventually drops to Fletcher on the edge, and he smashes a magnificent first-time half-volley into the roof of the net. He couldn't have caught that much sweeter.
73 minPeterborough look to respond again straight away and almost get a couple of chances to do so with the ball inside the West Brom area, but it never sits up for a go at goal and the visitors escape.
74 minSAVE! The hosts come forward again as a cross arrives from the right which Beautyman meets. He steers his header towards the far corner, but it lacks the power and Foster is able to get across to comfortably gather.
75 minShooting chance at the other end for West Brom as the ball is fed into the feet of Sessegnon, but he lashes an effort well over the crossbar.
77 minWest Brom are beginning to ask a few questions themselves now, but Peterborough don't have much to worry about if everything ends like this. Rondon collects the ball on the right side of the area, but slices his cross straight out of play.
78 minSAVE! Coulthirst cuts inside from the left flank and has a go, but his deflected, dipping effort is pretty comfortable for Foster.
80 minTaylor is the latest to go for goal from range, but this is a wayward strike that flies a long way off target.
83 minSHOT! This game is getting nice and open now. Peterborough come forward, with Samuelsen and Coulthirst exchanging a number of passes between each other before the latter finally finds space to shoot, only to drill an effort wide.
85 minJust five minutes remain here, and if we don't see a goal in that time it will be extra time for the second night in a row in the FA Cup. Both sides are pushing for a winner in normal time, though.
86 minOFF THE LINE! West Brom are so close to getting that winner as a long ball forward draws Alnwick off his line to beat Rondon to the ball. He can only clear it as far as Berahino, who controls his shot towards the unguarded net, but Bostwick reads it and makes a match-saving block.
89 minPETERBOROUGH SUB: The hosts make their first change of the night as Coulthirst is replaced by Souleymane Coulibaly.
90 minCHANCE! Here comes the pressure from West Brom! They sweep forward with pace and Berahino slides a pass down the right channel for Rondon. The striker goes for goal early with a first-time effort, but Alnwick makes the save with his feet.
90+1 minThere will be three minutes of stoppage time at the end of the 90 here.
90+3 minEND OF 90 MINUTES: Peterborough United 1-1 West Bromwich Albion
9.38pmAnd so to extra time we go! There has been 180 minutes of action in this cup tie, and still nothing separates these two sides, so we will have another 30! Peterborough can feel a little hard done by having performed very well in this match so far, only to be undone by a stunner from Fletcher to cancel out Taylor's opener. Both sides have created chances but, on the balance of play, Peterborough probably deserve to be in the fifth round here.
9.40pmIt remains to be seen how much longer Peterborough can keep up their work rate as they have already put so much into this game tonight. The hosts appeared to be tiring a little towards the end of that match, and now they have another 30 minutes to deal with.
91 min ETKICKOFF: West Brom get us back underway for extra time and immediately give the ball away, but Chester is there to make an important challenge.
93 min ETMcClean still looks full of running at the moment. He is up and down the left flank and looks to be West Brom's best bet of winning this game at the moment.
94 min ETTaylor takes the ball all the way from the right touchline to the middle of the field before finally laying it off to Coulibaly, but it is a dreadful effort from the sub, who lashes a wild strike well off target.
96 min ETFletcher looks to add another stunning strike to his CV for tonight as West Brom come forward through Gardner, Rondon and McClean. The latter squares it to his captain, but this time his long-range effort is well blocked.
97 min ETThese Peterborough players are just throwing their bodies in the way of everything here. A low pass into the box finds Rondon, but he can't make anything of it with Bostwick in his way. He drops it back to Gardner, but the midfielder's low strike is blocked.
99 min ETCHANCE! Samuelsen has been brilliant tonight. He goes on another jinking run into the box, beating a couple of defenders with great skill before lashing his strike over the top from the edge of the area.
100 min ETPETERBOROUGH SUB: Another change from the hosts as Marcus Maddison replaces Beautyman.
101 min ETAnother magnificent block from Bostwick as he throws himself in the way of McClean's effort after the winger had cut in from the left flank. Bostwick has put in a titanic performance tonight.
103 min ETCLOSE! Rondon suddenly sparks into life, chesting down a ball forward before turning and firing a low strike just a yard or so past the post.
105 min ETRondon gets into a good position down the left flank and appears to be in a dangerous position, but who is there to halt him - Bostwick. What a performance from the defender.
105 min ETHALF TIME IN EXTRA TIME: Peterborough 1-1 West Brom
9.58pmStill nothing to separate the two sides with just 15 minutes remaining now! Both teams had yet another sight of goal in that opening 15 minutes of extra time without troubling the scorers, and if there are no further goals in the coming quarter of an hour then we will be going to penalties!
106 min ETKICKOFF: Peterborough get us back underway for the second half of extra time, and they have made their third and final change, with Erhun Oztumer replacing Taylor.
107 min ETWe have seen a lot of long-range strikes fly off target tonight, and this is the latest of those as Rondo powers a strike well wide.
109 min ETYELLOW CARD! Forrester goes into the book for a poor challenge on Gardner. It was late and low, but perhaps more to do with fatigue than malice.
111 min ETPeterborough are still battling away here. There are a lot of tired bodies out there, on both sides, but both are still giving everything to the cause. This has been a really good cup tie - exactly what makes this competition so great.
113 min ETCLOSE! What a way this would have been to win it! Maddison collects the ball and drives forward down the right flank. He gets his head up and tries an audacious chip from outside the box while in full flight, and he almost pulls it off too, but the ball lands on the roof of the net.
114 min ETOFF THE LINE! My word, this Peterborough defence is just impenetrable at the moment! The ball drops to Gardner in the area and the West Brom midfielder strikes a clean volley with plenty of power towards goal. Zakuani, who is playing in his first start after three months out tonight, is there on the line to make a brilliant block, though.
116 min ETZakuani, understandably, is feeling this one now. He is one of many Peterborough - and, in fairness, West Brom - players who have put everything on the line in this match.
118 min ETJust two minutes remain in extra time for one of these sides to get a goal now! Remember, West Ham waited until the 121st minute to get the winner against Liverpool last night, so this one is certainly not over just yet!
119 min ETYELLOW CARD! Fox goes into the book late on for a foul on Berahino, and West Brom now have a free kick in a position very similar to the one that West Ham scored from last night...
120 min ETCHANCE! Ohhhh it should be the same outcome as last night too! Gardner swings a deep delivery into the area and Berahino is in space at the back post from close range. He looks certain to take West Brom through, but he gets the header all wrong and can't steer it on target. What a chance that was!
120+1 min ETFULL TIME: Peterborough 1-1 West Bromwich Albion
10.16pmPenalties it is then! West Brom really should have won it in the dying seconds there, with the last meaningful action of the match, while they also had one cleared off the line in that second half of extra time. It remained 1-1, however, and we will need a shootout to separate the two now!
10.17pmSEBASTIEN POCOGNOLI SCORES! *Peterborough 0-1 West Brom Calm from Pocognoli to start us off as he sends the keeper the wrong way.
10.18pmBOSTWICK SCORES! Peterborough 1-1 West Brom* It's been a magnificent performance from Bostwick, and he caps it off with a firm drive down the middle.
10.19pmBERAHINO SCORES! *Peterborough 1-2 West Brom He missed a chance to win it at the end, but no mistake from Berahino here as he tucks his spot kick into the corner.
10.20pmMADDISON SCORES! Peterborough 2-2 West Brom* No mistake from Maddison either. He sends Foster the wrong way to level things up again.
10.20pmGARDNER SCORES! *Peterborough 2-3 West Brom Yet again the keeper is sent the wrong way as Gardner calmly converts.
10.21pmSAMUELSEN MISSES! Peterborough 2-3 West Brom* Advantage West Brom! Samuelsen goes down the middle, but it is not a good penalty and Foster saves with his legs.
10.22pmFLETCHER MISSES! *Peterborough 2-3 West Brom Fletcher hands it right back to the hosts, though! It is dreadful from Fletcher as he drags his spot kick wide!
10.22pmFOX SCORES! Peterborough 3-3 West Brom* Foster guesses the right way and perhaps should save it, but Fox sneaks it past the keeper and into the corner.
10.23pmCHESTER SCORES! *Peterborough 3-4 West Brom Great penalty from Chester as he fires it right into the corner. Pressure back on the hosts!
10.24pmANGOL MISSES! Peterborough 3-4 West Brom Poor from Angol as he puts his penalty straight at the keeper!
10.24pmWEST BROM ARE THROUGH TO THE FA CUP FIFTH ROUND!
10.38pmHeartbreak for Peterborough as they crash out of the FA Cup on penalties following a magnificent display full of heart and character. They don't deserve to be out following that performance and, despite the result, can take a lot of positives from their display, but ultimately it is West Brom who will be in the last 16 facing Reading. Peterborough took the lead on the night through Taylor, but Fletcher levelled things up again with a magnificent half-volley. That proved enough to take the game all the way to penalties, where Angol saw his decisive spot kick saved to send the Baggies through.
10.40pmThat is all we have time for this evening! Thank you very much for joining Sports Mole for what was a classic cup tie tonight, albeit one that ended in disappointment for the lower league side. I will leave you with our match report, and be sure to stick around for reaction too. From me, though, it is goodbye for now!