The Canucks were headed to their second straight blowout loss Wednesday night after Rich Peverley scored his second goal at 3:39 of the third period, putting the Bruins ahead 4-0 for good.
Cory Schneider replaced Luongo, who struggled along with his teammates in Boston.
"We have two out of three with home-ice advantage, and that's what we've worked for all season," Luongo said. "We need to get playing the way we did in the first two games."
With Boston's win, the finals are now a best-of-three, with Game 5 in Vancouver on Friday night.
After an early goal by Peverley, Michael Ryder put the Bruins up 2-0 midway through the second period on an innocuous shot that somehow fluttered past Luongo. The puck might have deflected off the stick of defenseman Sami Salo, who played Ryder much too softly.
Boston took control 1:18 later during 4-on-4 play when Patrice Bergeron forced Keith Ballard's turnover behind the Canucks' net and chipped it in front to Brad Marchand, who easily scored over Luongo's left shoulder. The Boston crowd serenaded Vancouver with chants of "Luonnnngo! Luonnnngo!"
Luongo finally left after Peverley scored again early in the third period, with the Boston fans roaring as Schneider replaced him.
Luongo gave up all eight goals in Vancouver's 8-1 loss in Game 3, but declined coach Alain Vigneault's offer to pull him from that game.
Information from The Associated Press was used in this report.