The hosts opened the scoring after only 95 seconds as Chris Templeman set up Paul McManus, who finished from 10 yards, and Stuart Malcolm soon headed home a second.
As Dunfermline looked for a way back into the game, Forfar added to their tally in the 74th minute when Gavin Malin buried the rebound after Dale Hilson's shot struck the crossbar.
A dismal afternoon for Dunfermline goalkeeper Ryan Scully was compounded in the 82nd minute as he spilled Templeman's effort to allow Hilson to net Forfar's fourth.
East Fife replaced Dick Campbell's side at the foot of the table, despite picking up a point in a 1-1 draw with Stenhousemuir at Ochilview.
John Gemmell struck the opening goal for the home side after 23 minutes, netting from the penalty spot. However, Craig Johnstone equalised from close range just before the break.
Stranraer survived an Arbroath comeback to run out 3-2 winners at Stair Park. The hosts ran riot in a one-sided first half in which Martin Grehan bagged a double and Jamie Longworth also scored.
Former Dundee and St Johnstone striker Steven Milne pulled one back for Arbroath after 59 minutes and Alan Cook added a penalty at the death, but Stranraer held on.
Airdrieonians secured a deserved 3-1 home success against Brechin City. Martin Hardie and Liam Coogans had the hosts 2-0 before Andy Jackson struck in the 38th minute. But, as the visitors searched for an equaliser, Lewis Coult snatched a late third for Airdrie.