Eric Bailly gave the Red Devils the lead as he prodded home on the stroke of half-time.
And Swansea were in the game until the last 10 minutes when the Red Devils added three goals in four minutes.
Romelu Lukaku, Anthony Martial and Paul Pogba all scored to repeat the feat they managed last weekend against West Ham.
And the French midfielder is happy with the way United have started the campaign.
Pogba said: "We've had a very good result, 4-0 again with a clean sheet, We want to carry on on this road.
"We are more confident [than last season]. We know each other more.
"We work for each other and I think that is easier for us."
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United have won their opening two matches of the Premier League season and sit top of the table.
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