Treasury Security G. William Miller has given Chrysler and the United Auto Workers an ultimatum to agree on major cuts in contract benefits by 4 p.m. today or forget about further help, at least from the Carter administration, in obtaining the $400 million in federally backed loans the company says it needs to stay in business.

"We have no more time," Miller said yesterday after a meeting between Chrysler and UAW bargainers and officials of the Chrysler Loan Guarantee Board. "By tomorrow, we've got to have their best shot at this, or we'll just run out of time," the secretary said.

The board meetings again at 4 p.m. today, and the company and union entered negotiations round-the-clock last night, "trying to have something ready" by that time, union spokesman Don Stillman said.

Should negotiators fail to meet the loan board's deadline, the whole question of the requested federal aid to Chrysler probably will have to be deferred to the incoming Reagan administration, said Miller.

An unusually testy UAW President Douglas Fraser was more succinct. "I doubt seriously that Chrysler could survive" such a delay, Fraiser said, adding that "it wouldn't be fair to Reagan to expect him to make a decision" on Chrysler "after being in office only a few days."

Miller said the immediate problem is that the loan board "does not feel that the plan presented by Chrysler is adequate," in that it will not cut costs enough to enable Chrysler to survive even with the proposed $400 million in federally backed loans.

The plan "needs to be improved, it needs to be tuned up, certain gaps need to be filled," Miller said. He implied that the major gap is the UAW's apparent reluctance to make further financial concessions to the company.

"We have to be sure that we can achieve a revised labor agreement between the company and the UAW that will be satisfactory in terms of cost structure,"Miller said. He said he told both sides at yesterday's meeting that the board "needs to see adequate concessions" and that "the concessions proposed in the labor area by the company seem to be the minimal that would be necessary" to qualify for the $400 million.

"We urge the union and the company to sit down right now and to finish the process of collective bargaining" and to get back to the board as soon as possible before today's 4 p.m. deadline with an agreement, Miller said.

Fraser told reporters later that he understands Miller's position, but that he doesn't know "if we could negotiate to a conclusion his precise expectations."

In the current round of negotiations, the union already has offered the company a wage freeze amd other breaks that fall a little short of Chrysler's initial request for about $600 million in concessions between now and September 1982.

"But, now, apparently what is desired is an absolute cut in wages," Fraser said. he said the company is suggesting that the UAW eliminate its $1.15 an hour cost-of-living allowance, as well as take substantial cuts in recently won health and pension benefits.

Even if UAW executives agree to those concessions, "all of these things will be very, very difficult, if not impossible" to sell to the union's rank and file, Fraser said.

Another UAW official put it this way: "We're talking abut 40 years of collective bargaining going down the drain. It's very hard."

Whether an agreement can be reached by today's decline depends as much on "Chrysler's attitude" as it does on the union, Fraser said. He said the union might consider heavier benefit cuts if the company agrees to profit sharing as a way to help the workers recover.

But the company officials "have not volunteered, nor have they favorably indicated that they would go along with profit sharing," Fraser said.

In a related development, General Motors Chairman Roger B. Smith, suggested that his company might seek concessions similar to any the UAW gives to Chrysler.

"It's no good to settle with one [company] and to push the problem on to another. the UAW recognizes this. . . . The pattern is set and Chrysler will show the way," Smith said.

Asked to comment on those remarks, Fraser said terseley: "I don't want to talk about it."