Haaland's treble, remarkably his third in successive home Premier League games, took his tally since joining the champions in the summer to 17 in just 11 appearances in all competitions.
Foden also had a huge hand in a rampant City display which evoked memories of their famous 6-1 derby demolition of United at Old Trafford in 2011.
Foden began the rout after just eight minutes and United were a distinct second best thereafter, although Antony pulled one back and Anthony Martial also claimed two consolation efforts.
The nine goals were a stark contrast to Sunday's other Premier league match in which Leeds United, reduced to ten men, drew 0-0 at home to Aston Villa.
Pep Guardiola hailed Haaland's approach since signing. "He came (here) and realised, 'Oh, these guys, my mates, run like an animal and I have to do it too'.
"He said, 'I prefer to touch the ball five times and score five goals'. I don't like that, I want him to touch the ball more and more, so he became a football player who scores goals.
"I like that he's part of more situations, but don't forget his biggest talent is to put the ball in the net."
The result, which took City back within a point of the top of the table, both underlined the champions' brilliance and emphasised the size of the task still facing Erik ten Hag at United.
"I also told the players (it was a) lack of belief," Ten Hag said. "I've seen some highlights. It was obvious that we are defending not (on the) front foot, we let them play.
"We were, in possession, not brave enough, made technical skills mistakes, decision mistakes and then you get hammered."
A run of four successive wins had fuelled optimism and the visitors' hopes were also raised when it emerged key City midfielder Rodri was out injured and the hosts would line up with Manuel Akanji and Nathan Ake in central defence.
But City were so dominant the eighth-minute opening goal, swept in by Foden swept from a Silva cutback, seemed well overdue,.
Ilkay Gundogan then hit the post from a free-kick and Tyrell Malacia was also booked for pulling back Foden.
It was remarkable City had to wait until 34 minutes for their second goal which came when Haaland powered in a header from a corner.
Haaland then converted a superb pass from Kevin De Bruyne before turning provider with a similar ball Foden despatched.
United scored a fine consolation after the restart when Antony curled one inside the far post from 25 yards.
But normal service was soon resumed as Haaland powered in a shot from a Sergio Gomez cross.
Not to be outdone, Foden laid his claim to the match ball when he grabbed his third soon after, timing his run on to a Haaland pass superbly to fire past De Gea.
Cristiano Ronaldo was left on the bench throughout. "I wouldn't bring him in because we are 4-0 down out of respect for Cristiano, for his big career," Ten Hag added.
At Elland Road Leeds held on for a point after Luis Sinisterra was controversially sent off in the 48th-minute after failing to retreat from a free-kick and earning a second yellow card.
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