The League One side beat Aston Villa in the third round and Burn admits that the Cottagers will have to guard themselves against complacency.
"They'll be pressing hard and they won't let us have any time on the ball. Compared to the games against Norwich City in the Third Round, it will be very different to that," he told the club's official website.
"They'll probably go quite direct, using the channels, and they've got two big centre-halves. Last season I think they conceded the least goals out of all four leagues and I don't think the back four has changed much. They've got some very good players and squad-wise, I'd say they've got one of the best in League One.
"They've shown they can beat Barclays Premier League opposition in Aston Villa so they've got nothing to fear. They want to progress as far as they can in this cup so we've got to make sure we're on our guard and get a result."
Fulham have never won the FA Cup in their history, losing to West Ham United in the 1975 final.