PHILADELPHIA (AP) _ A federal judge has sentenced a former IRS agent to four years in prison for his role in a tax bribery case involving designer Albert Nipon, saying the sentence would have been harsher if the man had not helped prosecutors in the investigation.

U.S. District Judge Raymond J. Broderick also fined Meyer Weiss $100,000 and sentenced him to five years' probation after the completion of the prison term.

Weiss, 61, had pleaded guilty to bribery and conspiracy after being accused of accepting $160,000 in exchange for a favorable tax audit.

Assistant U.S. Attorney Terri A. Marineri said Weiss's cooperation with investigators could lead to two or three more indictments.

Already six former agents and four businessmen have been convicted or pleaded guilty in what U.S. Attorney Edward S.G. Dennis called the ''largest single bribe ever uncovered''in the Internal Revenue Service.

Nipon was sentenced to three years in prison for conspiracy and tax evasion and Edmond Costantini, a former IRS agent who also pleaded guilty, got a four- year sentence and $100,000 fine for conspiracy and bribery.

According to prosecutors, Weiss, an IRS agent for 35 years, found questionable expense items in the 1978 and 1979 personal returns of Nipon and his wife, Pearl, and in the design company's corporate returns of the same year.

Costantini acted as the middleman between Nipon and Weiss for a $200,000 bribe, prosecutors said.

Nipon is the head of Albert Nipon Inc., one of the world's largest women's fashion design firms. Pearl Nipon, who was not charged, designs clothes for the company.