The Chicago Cubs have completed their first four-game sweep in six years and shortstop Larry Bowa attributes the success to aggressive baseball.

The host Cubs defeated Pittsburgh, 3-1, yesterday for their fourth straight over the Pirates (fifth straight overall) to move within six games of the .500 mark. It was the Cubs' first four-game series sweep since June 1977, when they dominated San Francisco at Wrigley Field.

Craig Lefferts (1-3) earned his first major league victory while Jay Johnstone's leadoff homer in the fifth triggered a three-run rally for the Cubs, who handed Pittsburgh its sixth straight loss.

"We played aggressive baseball, especially on the base paths," said Bowa, who singled and scored in the fifth. "If we keep playing like this, especially against teams in our own division, there's no telling how far we can go."

The Pirates scored in the fourth when Jason Thompson singled, went to third on Jim Morrison's single and scored on Tony Pena's single, but the Cubs came back with three runs in the fifth.

Johnstone hit a 2-2 pitch into the right field bleachers off Lee Tunnell (1-4) for his third homer of the year and, one out later, Steve Lake reached on an infield single and scored when Bowa's single eluded center fielder Lee Mazzilli for an error. Bowa, who took second on the play, scored on Mel Hall's two-out single.