WASHINGTON (AP) _ Postmaster General Anthony Frank announced today that he is resigning after four years to return to private business.

Frank said he would leave on Feb. 28. During his tenure as head of the Postal Service, he pressed hard for increased automation of the mail, an effort that reduced the work force by 40,000 jobs and helped reduce costs to the agency.

''It has been a great privilege for this immigrant - for this first- generation American - to do some public service,'' Frank said.

He said he will return to San Francisco, where he lived before accepting this job, and will become chairman of Acrogen, Inc., a privately held biotechnology corporation.

''We don't think it's an exaggeration to say that Tony Frank has been one of the most effective postmasters general in history,'' said Norma Pace, chairman of the Postal Service's governing board. She said the board would begin a search for a replacement immediately but no deadline has been set.

Frank, 60, also drew positive reviews from Van H. Seagraves, publisher of Business Mailers Review, a Washington-based newsletter, and John Jay Daly, president of Daly Associates, Inc., a consultant for major mailers.

''We suggested he leave two years ago. ... It took him a little while,'' said Moe Biller, president of the American Postal Workers Union. Biller has clashed with Frank, particularly over the contracting out of postal jobs and over major reductions in the agency's work force in recent years.

But Seagraves gave Frank good marks, citing last summer's labor contract in which major unions accepted increased automation of postal operations.

Frank insisted on hiring an outside firm to monitor the agency's delivery performance. The results have sometimes embarrassed the agency, but also led to improvements, Seagraves said.

But Seagraves faulted Frank for not cleaning house when he took over in March 1988, saying he sometimes became a captive of the postal bureaucracy.

Frank has been frustrated by the long and complex system for raising rates, however, in particular the recent case in which the agency's proposed 30-cent rate was cut to 29 cents by the independent Postal Rate Commission.

And in addition to conflict with some union leaders, Frank has drawn strong criticism from consumer advocate Ralph Nader, who has called for Frank's departure for some time. Nader accuses Frank of paying too much attention to large business mailers at the expense of individuals.

Frank was president of First Nationwide Bank in California when he was chosen to head the postal service, taking office March 3, 1988.

Born in Berlin, Germany, Frank came to New York with his parents in 1937. He is a graduate of Dartmouth College, the Tuck School of Business at Dartmouth, and has also studied at the University of Vienna in Austria.