Marcus Johansson put the puck past Tim Thomas less than 1 1/2 minutes after the opening faceoff, but Boston’s Rich Peverley tied it later in the first period. Semin put Washington ahead for good with 1:17 left in the second on a power-play goal, his second score of the series.

The best-of-seven Eastern Conference series shifts back to Boston for Game 5 on Saturday. Game 6 will be in Washington on Sunday.

Washington was 25-0-1 during the regular season when leading after two periods, and Holtby continued that trend, inspiring repeated chants of “Holt-bee! Holt-bee!” from the red-wearing spectators. After things got rowdy in Game 3, the Capitals did it with discipline Thursday: The hosts were called for only one penalty.

It’s been a tight series all the way, with neither team leading by more than one goal at any moment.

Semin decided things Thursday by zipping a wrister from the left circle past Thomas, after Alex Ovechkin and Keith Aucoin set it up with assists.

That came on Washington’s third power play of the game. The Bruins had zero extra-skater chances through the first two periods, as the Capitals played precisely the type of quiet hockey that their coach, Dale Hunter, said he really wanted to see.

Washington wasn’t called for a penalty until there were just under 10 minutes left in the third period, when forward Mike Knuble — playing because Nicklas Backstrom was suspended — was sent off for holding. But the Capitals killed that off without allowing any shots, making the Bruins’ power play 0 for 12 this series.

Backstrom is Washington’s top playmaker, the player who scored the winning goal in Game 2 — and his absence showed at times. There was generally less creativity at the offensive end, and the Capitals couldn’t even muster a single shot on their first power play, which he usually runs.

Backstrom was lost for one game after a cross-check to Peverley’s face at the end of Game 3 on Monday drew a match penalty.

There was all sorts of verbal jousting during the series’ off days, Tuesday and Wednesday. Hunter — himself no stranger to mixing things up during his playing days — accused Boston of targeting the head of Backstrom, who missed 40 games during the regular season because of a concussion. Hunter said he thought Backstrom was only trying to protect himself against Peverley after taking repeated shots to the head from Boston.

Bruins coach Claude Julien responded that such claims were “ludicrous” and “ridiculous.”

What had been a plenty-of-defense, little-offense, relatively quiet series early — a total of four goals were scored in Games 1 and 2 — turned downright feisty and more wide-open in Boston’s 4-3 victory in Game 3.

So Game 4 reverted to the first style, thanks in large part to Holtby.

The Canadian is only 22 — 16 years younger than Thomas, who earned last season’s Conn Smythe and Vezina trophies — and has made a grand total of 18 regular-season starts in his career. But Holtby is playing in his first NHL postseason because of injuries to Washington’s top two goalies, Tomas Vokoun and Michal Neuvirth.

And the kid has looked like a grizzled veteran for all but Game 3. He stopped 72 of 74 shots in Games 1 and 2, then was terrific again in Game 4. Over the span of about a minute early in the second period, Holtby made three reflex saves to thwart Bruins chances.

Washington needed all of 82 seconds to take the lead, thanks to the tweaked top line of Ovechkin, Johansson and Brooks Laich.

Ovechkin and Bruins defenseman Andrew Ference got tangled up by the boards near mid-ice and both tumbled down, creating some space for Laich to collect the puck and rush unencumbered into the offensive zone. He bided his time before sliding the puck over to Johansson for a high one-timer past Thomas.

Not only was that the Capitals’ first shot of the game — it was the hosts’ only shot for the first 12½ minutes, making Thomas’ save percentage 0.0 for a while.

Later, Peverley broke in alone on the right wing, deked defenseman John Erskine into a belly-flop, and scored through Holtby’s legs at the 13:12 mark of the first period. That made it 1-1 — and meant that five of Boston’s seven series goals to that point were scored with Capitals defenseman Dennis Wideman on the ice.

But Holtby wouldn’t let another puck get by.

NOTES: Holtby’s save total was the highest for a Capitals goalie all season. … The Bruins outshot the Capitals 14-3 in the first period, and 45-21 for the game. … Boston had won six consecutive road games. … Knuble, who last appeared in a game on April 5, played on a line with Joel Ward and Aucoin. … Washington F Cody Eakin was assigned to Hershey of the AHL before the game. He had four goals and four assists in 29 games with the Capitals this season, making his NHL debut Nov. 1.

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