The Blades had fought back from 2-0 down to level at 2-2 through goals from Gus Hamer and Will Osula before the break, before Joao Pedro sealed his hat-trick with two more goals in the second period.
Pedro scored twice from the penalty spot, both spot-kicks conceded by Jayden Bogle, before sub Danny Welbeck capped victory with a fifth deep into stoppage time which squirmed through the grasp of Blades debut goalkeeper Ivo Grbic.
United can now at least put full focus on their Premier League survival bid, with Crystal Palace next up for the Blades on Tuesday night. Here's how we rated Chris Wilder's side as they tumbled out of the FA Cup...

13. Oli McBurnie 5
Replaced Osula but couldn't get a clear sight of goal despite some nice bits of link-up play Photo: Michael Regan

14. James McAtee 5
Came on for Brereton Diaz and showed some lovely moments of skill, including one to set Bogle free down the right after wrongfooting the Brighton defence. Almost scored a stunner with one of the final kicks of the game but his curling shot hit the crossbar and bounced out rather than over the line | Getty Images

15. Andre Brooks 5
Earned a big cheer as he entered the fray in place of Osborn and was neat and tidy as ever on the ball without having too much influence on the game | Sportimage

