Defeat at Old Trafford would have left Man Utd needing maximum points and an improbable goal difference swing to have any chance of staying up, but Tahith Chong put them 1-0 up in the 24th minute.
Chong then set up Zachary Dearnley to score United's second just before half-time.
After the break, goals from Indy Boonen and captain Joe Riley put the hosts 4-0 up by the hour mark.
United remain at the foot of the table on 18 points, but are now just a one point behind second-bottom Sunderland and three points off safety with two matches remaining.
Man United: 13 April v Man City (A) PL2
Sunderland: 15 April v Leicester (H) PL2