Krejci beat James Reimer with a shot from the faceoff circle after coming down the left wing.
The Bruins are 15-2 when leading a best-of-seven series 3-1. And Toronto is 2-12-1 in its last 15 games in Boston.
Rangers 4, Capitals 3
Girardi ripped a shot from above the left circle, off a feed from Derick Brassard, to give the Rangers the lead again with a power-play goal 59 seconds into the third. The advantage was created by Jason Chimera's interference penalty at the end of the second.
Stepan made it 4-2 at 6:02, scoring into a wide-open net at the end of a give-and-go play in front with Carl Hagelin, who had a goal and two assists.
Game 5 will be back in Washington on Friday before the series returns to Madison Square Garden on Sunday. The home team has won all four games in the first-round Eastern Conference matchup.
Kings 3, Rams 2, OT
The win put the defending Stanley Cup champions on the verge of surviving the first round, leading 3-2 heading to Game 6 in Los Angeles on Friday night.
Alex Pietrangelo scored on a wrist shot from the point with 44.1 seconds remaining in regulation and goalie Brian Elliott off for an extra attacker. That forced overtime for the second time in the series.
The Kings' Justin Williams scored in the final minute of Game 1, also in St. Louis.
Jeff Carter scored in the opening minute of the second and third periods for the Kings, who became the first road team to win in the series.