A bright start amounted to nothing as Morton took control to keep their five-point advantage at the top.
Peter MacDonald opened the scoring from the penalty spot after Fouad Bachirou was clipped by Callum Morris, the ball going in off the inside of the post.
Andy Barrowman had a penalty to get Dunfermline back into it late in the first half but his shot came back off the crossbar.
Michael Tidser doubled Morton's advantage at the start of the second half with a 25-yard volley and then provided the cross for MacDonald to head the third.
Tidser scored his second goal and Morton's fourth, with only a crumb of comfort for Dunfermline from a headed goal from Morris.
Cowdenbeath and Raith Rovers fought out a 1-1 draw at Central Park, with a late goal from substitute Greg Stewart earning the home side a point warmly welcomed, as the two teams below them in the table both lost.
Raith top scorer Brian Graham headed his team in front just after the interval, his 22nd of the season. It looked as if it would be enough but Stewart reacted swiftly after David McGurn failed to hold a shot.
Falkirk were worthy 2-0 winners at Dumbarton, Thomas Grant and David Weatherston both scoring in the first half.
It was probably never going to be Dumbarton's day after Nicky Phinn was injured in the warm-up and was replaced in the starting line-up by Stephen McDougall.
Falkirk top scorer Lyle Taylor made his presence felt in the first half, being denied by a brilliant show from home goalkeeper Stephen Grindlay on more than one occasion.
Grant put Falkirk in front after 25 minutes from Blair Alston's pass. Weatherston scored the second when he raced onto Taylor's flick, and the visitors comfortably saw off a late flurry from the home side.
Hamilton Academical had to rely on an injury-time goal for the second successive week to gain a point, in a 1-1 home draw with Livingston.
Steve May, who had looked dangerous, finally found the net after Livingston looked good for three points.
Andy McNeil had denied May early in the game and the striker shot wide in the second half after Liam Fox gave away possession.
Iain Russell opened the scoring 13 minutes from time in a penalty-area scramble but May stepped up to deny Livingston victory.
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