Berbatov scored twice and converted a late penalty, with Park Ji-Sung and Antonio Valencia adding United's other goals against the visitors who had Conor Sammon sent off late in the first half.
Chelsea's faltering title challenge suffered another blow when they were held 1-1 by neighbours Fulham in an early kick-off at Stamford Bridge, the home team registering their third successive league draw.
Juan Mata scored after 47 minutes for Chelsea, lashing in a left-foot shot from the edge of the box, before Clint Dempsey equalised nine minutes later with a close-range effort after good work by Bryan Ruiz.
United's second successive 5-0 victory - they also thrashed Fulham by the same score - allowed them to halve their goal difference deficit on City who started the day with a plus-10 tally over their neighbours.
The last four league matches for United have produced 16 goals for and only one against.
"Today's result pleases me considering the injuries we have had. We had 10 players missing and (defender) Jonny Evans was taken off at half-time," manager Sir Alex Ferguson told Sky Sports.
"In a close game I would have worried about that. We ended up with a back four that has never played together."
At the Hawthorns, West Brom became the first team to prevent City scoring in the league this season.
"Today was difficult," said City manager Roberto Mancini. "When you play a team who have all their players behind the ball at home, if you don't score in the first half it's not easy.
"It is impossible to score three or four goals in every game. There will be some moments in the year when you won't score."
Liverpool remained sixth with 31 points after being held to a surprising 1-1 home draw by bottom club Blackburn Rovers.
A Charlie Adam own goal - his second in a game against Rovers in successive seasons - put the visitors ahead late in the first half before Maxi Rodriguez equalised eight minutes after the break at Anfield.
Blackburn manager Steve Kean, under intense pressure from fans to quit all season, enjoyed the moment.
"It was a patched-up team because we have so many players injured and then we lost our goalkeeper [Paul Robinson] but Mark Bunn came in and made some great saves," said Kean.
"We have got a very young team at the moment but we are digging in and... I thought my lads were exceptional."
Blackburn narrowed the gap on their nearest rivals after 19th-placed Bolton Wanderers lost at home for the eighth time in nine league matches, going down 2-0 to Newcastle United.
It was a welcome win for Newcastle who ended a run of six matches without a victory thanks to second-half goals by Hatem Ben Arfa and Demba Ba.
Sunderland and Everton drew
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