NEW YORK (AP) _ ''A Chorus Line'' will keep kicking for four more weeks.

Broadway's longest-running show, originally scheduled to close March 31, will now end its lengthy run on April 28 after 6,137 performances, its producer Joseph Papp said Thursday.

''In my original announcement, I made it clear that if box office pressure warranted, I would extend the closing a few weeks,'' said Papp, head of the New York Shakespeare Festival. ''I am pleased that the public has rallied so splendidly. The extension will permit some 50,000 more people the opportunity to see the show.''

Business has boomed at the Shubert Theater box office. ''A Chorus Line'' is almost completely sold out through the end of the month, Papp said. Weekly grosses have jumped nearly $200,000 since Papp's Feb. 21 announcement that the musical would finally shut.

The 33 additional performances include two benefits. The final performance on Saturday evening, April 28, will be a benefit for the Shakespeare Festival, which produced the show. There also will be a special performance for ''Equity Fights AIDS'' on Sunday, April 8, in memory of Michael Bennett, the show's creator. Bennett died of AIDS in July 1987 at age 44.