The Flyers reduced the deficit in the best-of-seven series to 3-1, with Game 5 in Boston on Monday. The Bruins can clinch their first conference finals since 1992 with a victory.
Former Flyers star Mark Recchi scored twice for Boston, including the tying goal with 31.5 seconds left in regulation. Michael Ryder and Milan Lucic had the Bruins other goals.
Chris Pronger, Danny Briere, Claude Giroux and Ville Leino all scored for Philadelphia.
Gagne played for the first time since he broke a toe in Game 4 of Philadelphia's opening-round series against New Jersey.
Expected to miss this series, he managed an early comeback and heard a rousing ovation when he was introduced in pre-game introductions. Gagne got the crowd roaring 31/2 hours later on his first goal of the post-season.
The Flyers wasted a 3-1 lead and were deflated when Recchi scored the tying goal.
Recchi burned his former team when he scored against Brian Boucher off a faceoff in Philadelphia's end. It was his sixth goal of the postseason.
In Vancouver, Chicago captain Jonathan Toews scored three power-play goals and added two assists as the Blackhawks beat Vancouver and grabbed a 3-1 lead in the Western Conference semifinal series.
Toews took over the NHL playoff scoring lead with 18 points as Chicago moved within one win of their second straight trip to the Western Conference finals. The Blackhawks have won three straight after dropping the series opener at home.
Patrick Sharp also scored on the power play and Tomas Kopecky added a goal shortly after another penalty expired.
Chicago went 4 for 8 with the man advantage.
Brent Seabrook opened the scoring 18 seconds in, Dave Bolland scored into an empty net with 37 seconds left, and Antti Niemi made 26 saves for the Blackhawks, who can advance with a win in Chicago on Sunday.
The Canucks goals came from Daniel and Henrik Sedin, Kyle Wellwood and Alex Edler and Roberto Luongo made 27 saves.
