Semyon Varlamov made 28 saves, and Kyle Palmieri scored for the Islanders, the No. 4 seed in the MassMutual East Division. They outshot the Bruins 13-8 in the third period.
"When you can get a little bit of momentum and not give the other team a chance to get some momentum, that's huge," New York coach Barry Trotz said. "… I liked our whole game. They're a [heck] of a hockey team, there's no question. They're very talented, they've got some exceptional people on their team. I thought we played our game. We checked well, we competed at a high level, and we got things done."
David Krejci scored for the Bruins, the No. 3 seed. Rask made 30 saves.
Game 5 of the best-of-7 series is at Boston on Monday.
"I think it has been like expected," Rask said. "If you look at the style of hockey they play throughout the year, they play a hard game, heavy game, and there's not any real estate out there. So I think it's been kind of like everybody expected it. I think the physicality has just gotten more and more intense as the series has gone on.
"I think it's a tight series. Another tight game today. Like it's been in the previous games, time and space it's tight out there, and scoring chances are tough to come by. Like the game-winning goal again, it was one of those kind of bounces here and there, and it hits the back of the net. So it's 2-2."
Krejci gave the Bruins a 1-0 lead with a power-play goal at 3:57 of the second period on a rebound near the right side of the net for his first of the playoffs. The Islanders challenged that Brad Marchand interfered with Varlamov, but the goal was upheld after video review.