Tony Watt pulled St Johnstone level with his second goal for the club
St Johnstone eased into the League Cup last 16 after overcoming Forfar to beat their hosts to top spot in Group B.
John Baird gave the League One side hope of qualifying by scoring first, but goals by Tony Watt, David McMillan and Matty Kennedy put the Premiership team through.
Falkirk leapfrog Montrose to finish second after beating East Fife.
But their six points will not be enough to qualify as one of the four best group runners-up.
Anton Dowds put East Fife ahead, but Dylan Mackin scored twice after the break to earn the Championship side the win.
In the sole Group C match, Highland League outfit Cove Rangers humbled League One side Raith Rovers.
Daniel Park's ninth-minute opener and Jamie Masson's second-half strike earned Cove their first points in the competition.
The draw for the last 16 takes place on Sunday following the final group stage game between Heart of Midlothian and Inverness Caledonian Thistle.
Those ties will be played on Saturday, 18 and Sunday, 19 August.
