SAN FRANCISCO — The San Francisco Bay Area’s main commuter railroad resumed service on Friday afternoon after a four-day strike by transit workers, though the reprieve to its 400,000 daily riders could be short lived.

Apparently deadlocked in their negotiations over a new contract for 2,400 workers, the Bay Area Rapid Transit System, known as BART, and its two biggest unions agreed under pressure from the state to extend the previous labor contract for a month, thereby guaranteeing service at least through Aug. 4.

The two sides said that they hoped to reach an agreement before the new deadline, but remained far apart on key issues, including wages, state pensions and health insurance.