The Paisley side have made a sluggish start to life in the Championship under Ian Murray, however they are now unbeaten in this competition since July 2004, having won it in the following season when they were also promoted to the top flight.
Murray’s side took the lead on ten minutes as Callum Gallagher headed in a Cameron Howieson cross from the left and it was 2-0 midway through the first half when Stevie Mallan shot past Dunfermline goalkeeper Sean Murdoch.
The Pars blew a chance to get back into things when Joe Cardle hit the bar from close range shortly afterwards, and they were made to pay when Scott Agnew hit home the third before the break.
Lewis McLear and Keith Watson came close to adding to the scoresheet before Mallan fired home a fourth two minutes from time.
Murray said: “We showed the quality we have in our squad and scored goals at good times. As well as getting us through to a semi-final, it would be great if this game is remembered as being the one that kick-started our season.”
Dunfermline manager Allan Johnston said: “We had a big support here and we let them down with a really poor performance. No-one gets pass marks and that is disappointing on what was a great opportunity to get to a semi-final.”
Peterhead also reached the last four with a 3-0 win over Stenhousemuir at Balmoor, where Jamie Redman ended a game of head tennis by nodding in the opener on 17 minutes. Rory McAllister converted a corner with a header to claim the second on 38 minutes, and Steven Noble crashed home the third goal right on half-time.
Their success was not warmly welcomed by manager Jim McInally, who said: “I know football is about winning but I am disappointed as we were never in control of the game. We have played better and lost.”
McInally added: “The fact that we could be playing at Hampden, Ibrox, Almondvale or St Mirren Park will possibly brighten things up.”
Elgin City were denied a first ever semi-final appearance after going down 2-1 at Hampden to Queen’s Park, with a bright start the key to the Spiders’ success.
Craig McLeish drilled home via two deflections after just four minutes to have Queens up and running and, as Gus MacPherson’s side dominated, Tony Ralston crossed for Chris Duggan to volley home their second with 18 minutes played.
Elgin got into the game when Darry McHardy lashed home an Archie MacPhee knockdown on 50 minutes, with Kyle MacLeod denied an equaliser two minutes later when Bryan Wharton hooked his header off the line.
MacPherson said: “We got the job done, although we made it far more difficult than it needed to be. It is a cup run our players can be proud of but we know we will be the side everyone wants to meet in the semi-finals.”
Jim Weir was far from happy, and he said: “The first half was the poorest I have seen from an Elgin side in my time as manager. After 15-20 minutes I could have made three changes.”