Boston center David Krejci (46) celebrates his game-winning goal in overtime against the Toronto Maple Leafs. (Tom Szczerbowski-USA TODAY Sports)
Krejci 'tricks' Toronto, Bruins one win from advancing
Zdeno Chara had four assists for the Bruins.
Game 5 will be played on Friday at Boston.
The Bruins overcame a 2-0 first-period disadvantage to take a 3-2 lead in the second period on Krejci's second goal of the game. But Clarke MacArthur soon tied it at and the game went into overtime after a goalless third period.
The winning goal came after a wide-open overtime in which the Maple Leafs had their chances.
Patrice Bergeron also scored for the Bruins. Joffrey Lupul and Cody Franson also scored for Toronto.
During regulation time, Maple Leafs goaltender James Reimer stopped 33 of 36 shots. Tuukka Rask stopped 34 of 37 shots in the Boston goal during regulation.
Overall, the Maple Leafs outshot the Bruins 48-45.
The Maple Leafs started well and scored at 2:35 of the first period on a 27-foot wrist shot by Lupul, his third goal of the series, after Phil Kessel whirled behind the goal and put a pass in front from the corner.
Boston had the game's first power-play situation when Dion Phaneuf was called for tripping at 12:43 but could not take advantage. The Leafs took a 2-0 lead with an even-handed goal at 18:32 on 61-foot wrist shot from the right point by Franson, his first of the series.
The Bruins ended the first period with a man advantage after Leo Komarov was sent off for charging at 19:10.
The Bruins took advantage with a power-play goal by Bergeron at 32 seconds of the second period, a 17-foot snap shot to cut the lead to 2-1. Bergeron's first goal of the series followed Reimer's save on a 65-foot slap shot by Chara. Milan Lucic picked up an assist, giving him at least one point in all four games.
The goal put the Maple Leafs on the offensive and they hemmed in the Bruins in their own end for long stretches. But the Leafs' first power play of the game came up empty after Gregory Campbell was sent off for hooking former UNH star James van Riemsdyk at 8:57.
The Bruins came back to tie the game 2-2 at 12:59 when Krejci scored on a 32-foot wrist shot.
With Colton Orr off for elbowing Chara, Krejci scored again at 16:39 on the power play, snapping home a 41-footer for his fourth goal of the series.
But the Leafs responded at 17:23 with a goal by MacArthur, his first of the playoffs. He snapped home a puck that sat invitingly 30 feet away and directly in front of the Boston goal.
Penalties to Krejci for roughing at 18:12 and Campbell for slashing at 19:19 gave the Leafs a two-man advantage to end the second period and to start the third.
Boston withstood that and had their own power play early in the third when Nazem Kadri picked up a double minor for high-sticking at 58 seconds. The Leafs were able to kill off the penalties.
The Leafs went on the power play at 13:27 when Chara went off for high-sticking but with no result.
Reimer's faulty clearing with slightly more than a minute to play in regulation nearly presented the Bruins with a goal but the Leafs survived.