The top-seeded Capitals evened the first-round playoff series at two games apiece.
A four-goal first period saw the Caps dominate the Leafs, but just like in Game 3, the Leafs scratched their way back to make things interesting.
Auston Matthews' third-period goal cut the lead to 4-3 with eight minutes to go, and the Air Canada Centre buzzed with frantic anticipation of perhaps a fourth consecutive overtime game.
But T.J. Oshie scored his second goal of the game at 12:59 to give Washington a crucial insurance goal.
Toronto native Tom Wilson scored twice for the Caps, saved a goal and made head coach Barry Trotz look smart for promoting the winger to the third line. Tyler Bozak scored with less than a minute to go to make it close, but this time the Leafs weren't coming back.
Alex Ovechkin scored from his office on a bomb in the opening period, and the Caps now head home in a much better place mentally after opening the series with three consecutive overtimes games, two of which they lost to the No. 8-seeded Leafs.
The Caps were able to aggressively forecheck and force turnovers from the Leafs, which was the kind of thing most people expected to happen. Toronto looked disorganized defensively, especially in the first period, leaving Capitals players uncovered shift after shift. The Caps also tightened up their gap coverage, preventing many Leafs passing plays in transition that had been successful in the opening three games.