The win gives Calgary a commanding 3-1 lead with the best-of-seven Eastern Conference quarter-final series, with Game 5 set for Thursday in Calgary and Game 6, if needed, in Swift Current on Saturday.
It was the third game in the series decided in overtime. Calgary won Game 1 in OT while Swift Current took Game 3 in extra time.
Both teams finished 0-for-2 on the man-advantage.
Eetu Laurikainen made 26 saves for the Broncos.