A Ryan Giggs penalty after just three minutes set United on their way and then early second half goals from Wayne Rooney and Javier Hernandez put the tie to bed.
Hernandez made it four in the 66th minute when he grabbed a second after Aaron Hughes deflected his close range shot in, and although the Northern Irishman headed in a corner in the 77th minute at the other end it will offer Fulham little consolation, as they were lucky to only lose by three in a completely one-sided contest.
Fulham have built up an unwanted reputation for being a poor away side and they got off to a horrendous start which they never recovered from.
With less than 30 seconds gone Steve Sidwell gave the ball away allowing Wayne Rooney to run to the edge of the area. His shot was deflected out for a corner but from that resulting chance Hughes stupidly handled the ball when going up for a header with Chris Smalling, handing United a penalty after just 72 seconds.
Giggs took it and despite Mark Schwarzer getting two hands on it, the ball nestled into the corner of the net.
United then proceeded to dominate the rest of the half and if they were not so lethargic they could have scored a couple of goals.
Wayne Rooney and Nani had their best chances while they also should have had a second penalty in the 20th minute for handball, with Damien Duff this time guilty of using his arm to control inside the box.
The Londoners did have a shout for a penalty just on half-time though as Smalling seemed to bundle into Sidwell as a corner was floated in but referee Mark Clattenburg, taking charge of a United game for the first time since that now infamous game against Chelsea, blew his whistle for half-time rather than a foul.
Fulham were bad in the first-half and then truly awful in the second.
United killed the game off five minutes after the restart when Rooney turned Hughes on the edge of the box before firing home a smart low finish after being set up by a lovely pass from Anderson.
Two minutes later and it was three as Hernandez smashed home from close range. The defending from Fulham was again pathetic though as they failed to deal with crosses from Nani and then Rooney.
A fourth followed in the 66th minute as United were able to build relatively untouched. The ball was played into Hernandez inside the box - he turned and shot and the ball took a big deflection off Hughes and crept into the corner of the net.
The only sour note for United was that they continued their recent trend of conceding from a set-piece as Hughes headed home a corner from substitute Georgios Karagounis. A clearly embarrassed Fulham side didn't even dare celebrate.
Instead the game limped to a conclusion - Rooney had the ball in the net again but Hernandez fouled Hughes in the build-up and Fulham were lucky to return to London after conceding only four.
It was far too easy for United who were able to leave Robin van Persie on the bench for the entire contest. They move onto the fifth round, where hopefully the television viewers will be treated to a better spectacle if they keep their eight-run of live FA games intact.
