Oliver Wahlstrom had a goal and an assist for the Islanders, the No. 4 seed in the MassMutual East Division.
"He's been solid in there all year, both of them have been," Islanders forward Jordan Eberle said of Sorokin and Varlamov. "… It's impressive to see [Sorokin] obviously in his first playoff run. I know he's been through it in Russia, but to have poise and just his confidence back there and just his calmness, he makes big saves for us and he's done it all year."
Zach Aston-Reese scored, and Tristan Jarry made 22 saves for the Penguins, the No. 1 seed. Sidney Crosby did not have a point for a third straight game.
Game 5 is at Pittsburgh on Monday (7 p.m. ET; NBCSN, SN1, TVAS2, ATTSN-PT, MSG, MSG+).
"I think it was probably closer than the score, but I don't think it really matters at this point," said Crosby, the Penguins captain. "They played better and deserved to win tonight."
Sorokin made a stick save on Kasperi Kapanen's wrist shot from the right face-off circle at 59 seconds of the first period and got across the crease to deny Crosby near the left post at 5:32 of the second period.
The Penguins scored against Varlamov 3:22 into Game 2 and 2:01 into Game 3.
"We have confidence in him, but I think the whole bench said, 'Hey, he's dialed in, so we've got to be dialed in too,'" New York coach Barry Trotz said. "It was nice to play with a lead for once in this series. We haven't done that a whole lot. Real key for us."