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.