A federal judge here yesterday ruled that The Washington Post must continue to abide by provisions of expired, contract with the Washington Baltimore Newspaper Guild while negotiations for a new contract are under way.

The Post had maintained that it was no longer bound by the terms of the contract with the union that represents approximately 1,000 commercial, news and other employees at the paper since the agreement expired April 1, 1976.

The issue which brought the case before U.S. District Judge John H. Pratt was whether four employees who were discharged by The Post since the contract's expiration could take their cases to arbitration as the contract allowed.

The judge held that the terms of the contract remain in effect while negotiations continue for a new contract.

"It would do violence to the parties' scheme of coexistence if The Post were permitted to strip union members of their arbitration rights . . . by the simple expedient of prolonging negotiations regarding a successor contract," Pratt wrote.

When the contract expired in 1976, another union was seeking to represent the employees then represented by the Guild. After the Guild won that representation dispute, negotiations between the Guild and the paper began in October, 1976, and continue to this day.

Pratt said he found the newspaper's position on portions of the contract language "untenable." He said The Post "cannot be permitted to convert this dispute to its own advantage by attempting to alter the existing employment relationship."

He ordered the paper to "resolve all grievances" as required by the contract, including those arising out of the four dismissals.