Bargaining between the U.S. Postal Service and its two largest unions resumed after a four-day impasse. The contract between the Postal Service and the American Postal Workers Union and the National Assn. of Letter Carriers, which together represent about 580,000 workers, expires at midnight tonight. No disruption of mail service is expected, even if an accord is not reached, because federal law prohibits strikes by postal workers. The bargaining had been broken off in a dispute over a separate pact with the Mail Handlers Division of the Laborers International Union. Union officials said that a memorandum of understanding had been signed that satisfied their concerns about the mail handlers contract.