he Philadelphia 76ers made only two of 23 shots in the fourth quarter but converted 12 of 16 free throws down the stretch to subdue the Boston Celtics tonight, 102-90, and move within one game of the NBA's Eastern Conference championship.

The 76ers took a 3-1 advantage in the best-of-seven series, with Game 5 set Sunday at Boston Garden.

"We have three quarters of the cake, but we need that fourth piece before we can eat it," Philadelphia Coach Billy Cunningham cautioned. "This series isn't over yet."

Julius (Dr. J) Erving was once again the 76ers' stars as he ran circles around Larry Bird on his way to a game-high 30 points. Erving supplied 10 rebounds, three assists and two-blocked shots, although he couldn't get the ball down in that final quarter.

Erving was all over the court the first three quarters and had 27 points going into the final period to stake the 76ers to an 86-74 lead.

They missed their next 12 shots and the Celtics, with M. L. Carr leading the way, kept clawing and scratching, trying to get into the game.

Two baskets by Carr and a three-point play by Cedric Maxwell cut the Philadelphia lead to 88-81 with 7:44 left. Bobby Jones banked in an 18-footer for the 76ers' last field goal of the game with 7:18 to play. Their remaining 12 points came from the foul line.

The closest Boston approached was five points, on two occasions. A Carr free throw with 1:17 left made the score 95-90, but Maurice Cheeks was fouled by Tiny Archibald 20 seconds later and made two free throws for Philadelphia.

Bird Celtic leader with 19 points and 13 rebounds, fumbled the ball out of bounds as he maneuvered for a shot on Boston's next possession, and Lionel Hollins was fouled by Chris Ford. Hollins made two free throws to seal the outcome.

"The ball just wouldn't stay in the basket in that fourth period," said Erving. "We got it up enough times."

A mirror fell on Carr in the Celtic dressing room after the game. He was cut on the right shoulder and the back of the head and 14 stitches were needed to close the wounds. He is expected to play Sunday.

Until the fourth period, the Sixers had the game pretty much the way they wanted it. The Celtics were vircually always on the defensive, reacting to the 76ers, trying to come up with a move to slow them down. Nothing seemed to work.

Erving waited until the second half to turn it on in the third game of the series Wednesday, but he was hot from the beginning tonight. He amassed 11 points, six rebounds, two blocked shots and three assists in the first quarter, and the 76ers grabbed a 28-21 lead.

Three times, Erving drove around Bird for dunks so quickly that the celebrated Boston rookie could do nothing but grab at his jersey as he went by.

Both Caldwell Jones and Darryl Dawkins were in foul trouble so the 76ers went to their substitutes. Every team should be so lucky.

Reserve Bobby Jones came up with 14 points, seven rebounds and give blocked shots, in addition to putting the defensive clamps on Bird. Steve Mix scored all eight of his points in the second quarter when the 76ers were moving to a 12-point lead.

The Sixers were up by only 45-43 when Mix led them on a 12-2 drive. He had six of the points, all on fast-break opportunities.

The Celtics rallied at the tag end of the half, howevr, to cut the advantage to 59-52.

Erving scored 12 points in the third period, including a three-point shot, and the sellout crowd of 18,276 started rocking.

The Celtics now have lost five of five in the Spectrum this season and if they beat the 76ers Sunday will have to return here for the sixth game Wednesday.