Linfield gained revenge for their Co Antrim Shield final defeat by Ballymena with a thumping win at the Showgrounds.
All the goals came in the second half, starting with a Sean Ward tap-in from Ross Gaynor's cross.
Gaynor netted a penalty after Paul Smyth was fouled by Tony Kane, who was sent-off for two yellow cards.
Smyth fired in the third before Andrew Waterworth rifled into the top corner to seal a win which keeps the Blues nine points behind leaders Crusaders.
It was a lacklustre first half with United keeper Alan Blayney keeping out Mark Stafford's backpost header.
Ballymena were bossing the game when Linfield scored the opener on 63 minutes but hopes of a comeback fizzled out with Kane's dismissal and the resultant penalty three minutes later.
Smyth celebrated signing a contract extension this week at Windsor Park by fizzing a low shot past Blayney.
The best was kept to last as Waterworth arrowed his volley beyond Blayney and completed a 13-minute goal blitz from the visitors.
What they said
Linfield captain Andrew Waterworth said: "The pitch was a bit of a mess but I think we were on top in the first half.
"Ballymena started the second half strongly and the opening goal came at a vital time as the game opened up.
"There's a lot of points to be won and dropped in the run-in and it's about what we do."