With the bargaining deadline only four days away and no major issues resolved so far, a suit was filed yesterday to block any strikes or slowdowns by the nation's 550,000 union postal workers.

The Public Service Research Council, a 900,000-member organization that opposes what it calls "union control of government," asked U.S. District Court here to enjoin union leaders from threatening or calling a strike.

In addition, it asked the court to force the Postal Service to fire and not reinstate any strikers.

The group, which raises about $3.5 million annually through third-class mail solicitations, also sought to bar implementation of contingency plans for modified service in case of a strike.

Current contingency plans under study by the Postal Service include possible use of federal troops to handle the mail, suspension of home deliveries or Saturday service, and temporary elimination of third-class mail.

Federal law bars postal strikes and calls for dismissal of strikers, but it was not enforced in 1970 during the last mail strike, when 200,000 postal workers walked out and later went back to their jobs under amnesty negotiated as part of the eventual contract settlement.

This year some union leaders have threatened "no contract, no work," although both sides at the bargaining table express hope for a settlement by the Thursday midnight deadline.

David Y. Denholm, president of the Vienna, Va., based conservative group that filed the suit, said its purpose was to force the Postal Service and the unions to obey the letter of the law.