A strike by production and maintenance workers at a Tennessee nuclear weapons plant and research laboratory operated by Martin Marietta Corp. entered its fourth day yesterday, and the federal mediator assigned to the dispute said he had no immediate plans to call representatives from management and labor back to the bargaining table.

"I think we need to let the dust settle for awhile," said Lonnie Stokes, a federal mediator based in Knoxville.

Union members at the Y-12 nuclear weapons production plant and the Oak Ridge National Laboratory in Oak Ridge, Tenn., formed picket lines Saturday afternoon when delegates from 17 unions represented by the Atomic Trades and Labor Council voted 23 to 20 to reject a contract offer from Martin Marietta Energy Systems Inc., a subsidiary of the Bethesda corporation.

Both facilities remain open, but production has come to a halt at the Y-12 plant. About 3,200 of the 7,200 workers at Y-12 and 900 of the 5,000 workers at Oak Ridge National Laboratory are on strike, according to Martin Marietta spokeswomen Carol Grametbauer.

Grametbauer said the strike's impact was much greater at the Y-12 plant because striking employes there were involved with primary production tasks, while those at Oak Ridge National Laboratory were involved mainly with maintenance.

The Y-12 plant, the only one of its kind in the nation, fabricates uranium components used in all U.S. nuclear weapons systems, according to Department of Energy spokesman Jim Alexander. The strike will not have any immediate effect on the nation's nuclear stockpile, Alexander said, but could pose a serious problem if it continues.

Martin Marietta has been negotiating a new contract with the unions since April. Their original three-year contract expired last week.

Negotiations have foundered on Martin Marietta's proposal to make craft classifications more flexible. Management officials have called the move a productivity issue, arguing that consolidating the responsibilities of workers in different craft groups would render the facilities more efficient. The unions oppose the measure as a "cancer that will spread to other areas," according to ATLC President Bob Keil.

Members of the ATLC's bargaining committee had approved a contract with Martin Marietta before negotiations expired Saturday, but a group of 43 union delegates later voted narrowly to reject it. On Sunday afternoon, the proposed contract failed ratification by the union's general membership by a vote of 1,479 to 1,454.

Grametbauer said the two sides were very near a new contract agreement, but Keil described negotiations as "pretty much at an impasse" and unlikely to be resolved in the immediate future.