Malcolm scored his first three minutes from half-time when he managed to get in behind the East Fife defence to pick his spot.
He struck again early in the second half before completing his hat-trick with an eight-yard shot to take the game out of East Fife's reach.
Brian Gilmour scored the fourth before Buchanan pulled a goal back for East Fife, who go into the relegation play-offs.
Airdrieonians sent Arbroath into League Two, with goals from Caolan McAleer and Liam Coogans sealing a 2-0 win at the Excelsior Stadium.
Stranraer snapped a three-match losing streak to beat Forfar Athletic 3-1 and clinch a spot in the play-offs.
Gavin Swankie gave Forfar an early lead, but the tide was turned by goals from Sean Winter, Scott Rumsby and Frank McKeown.
Stenhousemuir beat Brechin City 3-1 but it was all to no avail with Ayr and Stranraer both winning.
John Gemmell's two penalties and a goal from Ben Greenhalgh swept Stenhousemuir to victory, with Alan Trouten replying for Ayr with a penalty.