Propelled by four home runs, two of them by Mike Schmidt, the Philadelphia Phillies regained first place in the National League East tonight by sweeping the Montreal Expos, 9-5 and 5-0.

An extra shot in the arm, and possibly the major development of the night, was provided by right-hander Marty Bystrom, who pitched a five-hitter for his first shutout in three years and first complete game in 15 months.

Bystrom was making his first start since Aug. 20, when he was placed on the disabled list with a strained right elbow. He was so impressive in a 112-pitch performance that a few reruns in the next two weeks could give the Phillies the edge in this frenetic four-team scramble.

Montreal fell into third place, 1 1/2 games behind the Phillies and a half-game back of Pittsburgh, with St. Louis now 2 1/2 from first.

"Marty was awesome after not pitching for a while," Schmidt said. "Did you ever see a more slump-ridden 5-0 victory in your life? We gave Marty two big swings and he took it from there."

Three Montreal pitchers got 11 strikeouts in the second game, with Schmidt striking out his last three times and cleanup batter Joe Lefebvre going down swinging all four at bats. But homers by Schmidt and Len Matuszek provided all the runs Bystrom needed.

Schmidt hit his 36th beyond the 408-foot marker in center--almost a duplicate of No. 35 in the opener--after right-hander Ray Burris walked Juan Samuel in the first inning.

Matuszek, buried in the minors in Pete Rose's shadow for several seasons, hit a three-run homer, his first in the majors, following singles by Bystrom and Samuel in the third.

"I felt strong, both on the mound and at the plate," said Bystrom, who also had a double for one-third of the Phillies' hits. "I went out to try to give them six good innings, but I didn't throw that many pitches, my control was good and everything just went the way I wanted it.

"I changed a lot of things in my delivery, smoothed a lot of things out and took a lot of strain off my arm. The layoff helped me work things out, because I had picked up a lot of bad habits and I had no time to work on them going from game to game."

Before Bystrom won his first game since Aug. 12, Steve Carlton benefited from 14 hits to gain his 298th success in the majors and first since Aug. 15.

Carlton threw 140 pitches in eight innings and struck out eight raising his record total to 3,683. Although Carlton was not impressive and trailed, 3-0, in the third inning, the Phillies got four runs in the third off Bryn Smith and four in the fourth against Greg Bargar to give Carlton a big nest egg.

At one stage, the Phillies put 10 straight batters on base, but scored only four times because the streak overlapped the second and third innings.

In the second, Philadelphia went scoreless despite a hit batsman and three singles. A double play wiped out the hit batter and, on the third of three straight singles, Greg Gross was thrown out at home by right fielder Mike Vail.

Carlton, Joe Morgan and Matuszek singled in the third to extend the Phillies' hit streak to six, but they still would not have scored except that center fielder Andre Dawson's error permitted Carlton to jog in.

Smith hit Schmidt in the back on a 3-2 pitch to refill the bases. Lefebvre's single reduced Montreal's advantage to 3-2 and Smith finally was asked to leave. Third baseman Tim Wallach booted a potential double play grounder to tie the score and Gross singled Schmidt home while becoming the 10th straight batter to reach safely.

Bargar escaped further trouble that inning, but the Phillies finished him in the fourth. Carlton walked, Morgan hit his 14th homer, over the wall in right, Matuszek doubled and Schmidt cleared the wall in center.

A two-run double by Manny Trillo cut the Expos' deficit to 8-5 in the seventh and Dawson just missed homering on a ball that Gross caught in deep left.

"That was the biggest break for us," Schmidt said. "It would have been 8-7 and who knows. These were two important games and I'm happy we won them, but we have to start again tomorrow like we didn't win any."

The Expos know that feeling well.