Ronald Koeman's side needed extra time to ovecome third-tier outfit Cornella in the third round, but they eventually got the job done thanks to goals from Ousmane Dembele and Martin Braithwaite in a 2-0 win.
Here, Sports Mole looks at how Barcelona could line up against Vallecano, who knocked Elche out in the third round.
Barcelona will welcome their wantaway captain Lionel Messi back for this match, with the Argentine returning to the fold after serving a two-game suspension for his sending off against Athletic Bilbao in the Super Cup.
Messi boasts 17 goals against Vallecano and could be thrown straight back into the first XI in place of Antoine Griezmann - who is due a rest before Barca face Athletic in the league this weekend - and the 33-year-old is in line to start on the left as Braithwaite leads the line once more.
Koeman will be sure to put out a strong team capable of advancing to the next round, but Trincao will still be hopeful of earning a start over Ousmane Dembele, with Philippe Coutinho and Ansu Fati still sidelined.
Sergio Busquets is suspended for Sunday's game with Athletic so it would be a surprise to see him left out of this one, and the veteran Spaniard will likely be supporting Miralem Pjanic and Puig in Koeman's midfield three.
Sergino Dest was an unused substitute against Elche after shaking off injury concerns, but Oscar Mingueza should continue here after Dest was ruled out of this game with an ongoing thigh problem, while Junior Firpo will look to line up at left-back over Jordi Alba.
Ronald Araujo has enjoyed an extended run in the team in the absence of Gerard Pique and should earn another start here, but Samuel Umtiti's place is in jeopardy with his compatriot Clement Lenglet hoping to feature from the off.