PITTSBURGH — As the Islanders head home with soft smirks on their collective faces, they can look in the rearview mirror and see the rest of the NHL left slack-jawed.

That’s just the way they like it, and Islanders completed their sweep of the Penguins with a 3-1 win in Game 4 of their first-round series Tuesday night, wrapping things up with one more tidy performance.

Under the stewardship of first-year head coach Barry Trotz, they played a defensive game that sucked the life out of the Penguins, utterly frustrating a team lined with championship pedigree. The Islanders suffered very few down moments, very few weak spots, and showed nothing but more group resolve.

That’s the story of their season as they keep proving people wrong.

“That’s been a little bit of the storyline here, and we’re used to it,” captain Anders Lee said. “We’re used to it being in our room. It’s not just this year, it’s kind of always been that way. This year, we’ve really taken it on and run with it.”

This was the franchise’s second postseason series victory since 1993, now part of a historic turnaround led by team president Lou Lamoriello. They will wait to face the winner of the Capitals-Hurricanes series, with defending champion Washington holding a 2-1 lead.

It surely would be pretty interesting to watch Trotz return to play the Capitals, with whom he won the Stanley Cup last season. It was when he faced them for the first time with the Islanders, Nov. 26, that he said, “You’ll have to come through the f–king Island” if they wanted to win again.

But that is a bit down the line. For now, the Islanders were just giving themselves a minute to realize their ride is far from over.

“I came into this team and talked to the guys straight in the summer when I got here, and a lot of the guys said right away we’re going to go far,” said goalie Robin Lehner, who was rock-solid again in making 32 saves as his personal and professional bounce-back year continues. “I think that just grew during the season as our system started to jell and we started to play together.

“There was not one single team in this league that crushed us, that we couldn’t play against — like Tampa [Bay], or Boston, whatever. We have a good team. We’ve been overlooked a little bit.”

Whichever team the Islanders face in the next round surely would be a challenge, but they are now in the midst of a wide-open Eastern Conference after the Presidents’ Trophy-winning Lightning got swept by the wild-card Blue Jackets. With the Bruins and Maple Leafs beating each other up, the Capitals might be the favorites, but it’s far more open than expected.

What has become crystal clear, after the Islanders suffocated the Penguins and outscored them 14-6 in the four games, is that they are for real.

They didn’t get flustered when the Penguins got out to an early lead for the second straight game, this time Jake Guentzel scoring just 35 seconds in. It came on a terrific pass from Sidney Crosby in the corner, the first and only point of the series for the Pittsburgh captain.

Then, just as in Game 3, the Islanders’ best skater, Jordan Eberle, needed very little time to tie it up, this instance coming 94 seconds later when he finished a 2-on-1 for his fourth goal in as many games. The Penguins sagged, and Brock Nelson was able to get his third of the series at 18:06 of the first for what would stand as the game-winner.

From there, the Islanders did what they do best — locked things down. They stifled Crosby. Evgeni Malkin was a non-factor. Kris Letang looked lost.

Maybe the Penguins are entering the twilight of their decade-long run. And maybe the Islanders caught them at a good time.

Or maybe the Islanders are showing that great coaching and a team-first attitude really does go a long way.

“Just a group effort,” Trotz said, “which you wouldn’t expect anything different from the Islanders.”