Davis finished with 18 points, nine of which came in the fourth quarter. Nate Robinson also came off the bench for 12 points, half of which were in the last period.
It essentially creates a best-of-three series. Game 5 is Sunday night again at TD Garden. After that, the series will end in Los Angeles.
On Tuesday, Davis had eight points in the last quarter of a 91-84 loss, but the rest of Boston's bench was held scoreless in the period. It was a much different story in Game 4, as Boston held a 36-27 overall scoring advantage in the last period. The Celtics had 21 points from their reserves in the final quarter.
"I just felt like a beast. I felt like I couldn't be denied," said Davis, who went 7-of-10 from the field. "I have to seize the moment here."
"They're a great scheming team," Bryant said. "They have a strategy in place, and they execute extremely well. I feel pretty comfortable. I wasn't pleased with the way I took care of the ball tonight. I thought I did a horrible job of that, but it's a great defense."
Davis was monstrous on the boards, muscling in for a put-back bucket with nearly 8 1/2 minutes left. He was fouled by Lamar Odom and completed the three-point play as the pace went up-tempo and in Boston's favor. That ended the 9-0 push for a 71-64 lead. Robinson jumped on "Big Baby's" back as Davis roared with approval of the change in momentum.
Davis didn't seem to worry about the extra weight on his back.
"You didn't even notice," Robinson said. "We're like Shrek and Donkey. You can't separate us."
Boston continued to move the ball with crisp passing and Tony Allen drove into the lane before dishing to Rasheed Wallace, who drained a three-pointer from the top of the arc for his lone field goal of the night. That provided a 79-70 difference at the midway point of the quarter.
Every time the Lakers tried to come back, Boston's gritty reserves had an answer. Robinson darted into the lane for a bucket for an 83-74 lead. Two free throws from Davis widened the margin to 85-74 with under four minutes left.
Davis and Wallace played better than 10 minutes in the last quarter, while Pierce and Garnett got extended rest time.
"They were fine. We were cheering," Boston coach Doc Rivers said about the decision to keep his bench players in the game. "This is a good team. I don't think guys really care...that's why we're here. (Rajon) Rondo and all of them, they were begging me to keep guys in. 'Don't take them out! Don't take them out!' It was great. That was the loudest I've seen our bench, and it was our starters cheering from the bench. I thought it was terrific."
Pierce hit a step-back jumper with over two minutes left and later plowed over Bryant for a layup and completed the three-point play for a 90-81 cushion with 1:17 to go.
Bryant made three from the charity stripe after being fouled, but Rondo made the back-breaking play to seal the win. Bryant drove right baseline, but was blocked off. His pass to Odom beyond the foul line was picked off by Rondo, who coasted in for a layup with 31.4 seconds left and a 92-84 advantage.
After the game, Lakers coach Phil Jackson didn't want to comment on his defensive strategy for guarding Davis or Robinson.
"I don't have any comment, either," Davis said laughing. "If Phil Jackson doesn't have a comment, then I don't have a comment."
The Lakers were short another offensive option as center Andrew Bynum didn't play in the second half due to a sore knee.
"We want Andrew out there as much as possible, but it's just a matter of how he's feeling and how much he can do," Gasol said. "But I think we have enough in our bench and in our team to be able to be aggressive and productive out there."
It was a back-and-forth battle for most of the first quarter, but both teams couldn't find a rhythm offensively. Boston was able to grab a 19-16 lead at the end of the period thanks to a three-pointer from Robinson.
Robinson canned another three-ball early in the second for a 24-21 Boston lead, but Bryant helped the Lakers surge late in the stanza, scoring eight straight points for his team. It was part of an 11-4 burst, as the visitors opened a 39-33 advantage following Odom's layup with just over three minutes left.
A layup from Ron Artest widened the gap to 43-35, but good defense from Rondo helped the Celtics get close again. He poked the ball away from behind on a driving Bryant and was the recipient of a long pass from Pierce, as the play ended in a breakaway layup for a 43-40 difference in the final minute.
Gasol blocked a three-point try from Ray Allen, but Garnett corralled the loose ball and sank a jumper from the left wing as time expired for a 45-42 score at the half.
It remained tight throughout the third quarter, but Davis' put-back brought the Celtics within 62-60 going to the fourth.
The Celtics held a 54-34 advantage of points in the paint and also a 16-8 edge on the offensive glass...The Boston win ensured the Celtics still has a chance to end the 2008 Finals rematch with a similar result. They beat the Lakers in six games...Wallace picked up a technical foul with 7:25 left in the game after being called for a foul despite cleanly slapping the ball away from Bryant. He and teammate Kendrick Perkins are one more technical away from a suspension...Rondo and Odom each had 10 points...Ray Allen finished with 12 points...Of the previous 25 times the Finals have been tied 2-2, the winner of Game 5 has gone on to win the series 19 times. The last time the Finals were tied 2-2 and the Game 5 winner didn't win the series came in 1994 when the Rockets lost Game 5 to the New York Knicks, then won Games 6 and 7 at Houston to win the title...The Celtics outscored LA on second chance points, 20-10. Over the first three games, the Lakers outscored Boston, 43-20 in second chance points...Boston held a 36-18 overall edge in bench points in Game 4.