The Regals broke open a close game by outscoring the Red Devils 4-1 in the third period en route to a 7-3 win in Game 4 on Saturday night.
The one-game showdown for a berth in the league championship series against the Charlottetown Abbies will be played at the Pownal Sports Centre on Monday at 7:50 p.m.
At the Jacques Cartier Arena, Nathan Handrahan (2), Brendan Smith (1-1), Robbie Fraser, Brett Aylward, Carson Adams and Ryan McNeill scored for the host Regals, who entered the playoffs as the No. 4 seed. Jake Getson (2), Calvin McRae, Jason Gallant, Evan Millman, Curklan Fraser, Jeff Gaudet and William Stewart assisted.
For the top-seeded Red Devils, Patrick Cummiskey (1-1), Jordan Hambly and Harrison Hayden dented the twine. Jackson Atkinson (2), John MacKenzie and Sam Hughes drew assists.
The Red Devils led 2-1 after the first period, but the Regals bounced back in the middle frame to take a 3-2 advantage into the second intermission.
Kelton Ellis earned the goaltending decision over Matt Derry.