United booked their place in the fifth round draw with a deserved and ultimately comfortable victory over Championship side Derby.
This was a much improved performance from Louis van Gaal's side which will ease the pressure on the Dutch boss who saw his side take the lead through Wayne Rooney's exquisite finish- his sixth goal in as many games.
George Thorne equalised against the run of play for Derby but United were the better side after the break as well-taken goals from Daley Blind and Juan Mata sealed our place in Sunday's draw.
United started well and the inspirational Anthony Martial went close with an early effort over the bar before the Reds took the lead on the quarter hour mark.
The goal came in superb style, too as Rooney picked up the ball from Martial to cut inside and curl home a sumptuous strike into the far corner.
With the Championship promotion chasers content to sit back and grant United possession, van Gaal's sharp-looking side continued to probe and almost doubled their lead as Martial was snuffed out by the Derby defence after deft link-up play with Rooney.
United were totally dominant so it came out of the blue, therefore, when Paul Clement's side equalised with their first attack of the match eight minutes before half-time.
Chris Martin picked out Thorne with a neatly lofted through pass and the midfielder took a touch before rifling home an excellent finish.
Stephen Warnock cleared from under his own bar and Chris Smalling thwarted Martin with a superb block at the other end in the last action of the half.
Mata nodded wide from Jesse Lingard's deflected cross and Fellaini also went close with a header, while Nick Blackman lashed over the top after a rapid counter attack from the hosts.
United went back ahead on 65 minutes as Blind made an unexpected burst into the box to slot in an excellent finish from a Lingard cross after a superbly-crafted move.
That goal seemed to deflate Derby and Mata then killed the game off with a late third- a simple driven finish across the keeper after Martial had beaten his man for the umpteenth time on the left.
Overall team performance: 7.5/10
United Faithful Man of the Match: Anthony Martial
A special mention must go to the 5,471 visiting United fans who never stopped singing all night and played their part in helping the team home.