Backup Glen "Big Baby" Davis scored half of his 18 points in the fourth quarter, leading the Celtics bench as it pulled away from the Los Angeles Lakers to win.
The Celtics' win guaranteed them a trip back to Los Angeles and averted a 3-1 deficit that has never been overcome in NBA history.
Kobe Bryant scored 33 points and Pau Gasol had 21 for the Lakers.
Paul Pierce scored 19 points, Kevin Garnett had 13 and Ray Allen bounced back from a seven-quarter shooting slump to score 12 points for Boston.
If Los Angeles wins the series, the Lakers would have their 16th NBA championship.
They'll play Game 5 in Boston Sunday, before heading back to L.A. for Games 6 and 7, if necessary. Sunday night's game is scheduled for 5 p.m. PT on ABC7.
The Associated Press contributed to this report.
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