The administration yesterday proposed a 4.8 percent pay increase for senior government executives whose salaries have been frozen for four years. The rise, which would cost $90 million a year, was proposed in an effort to halt an accelerating "brain drain" and raise plummeting morale.

The idea drew quick support from senators in both parties, led by Republican Ted Stevens of Alaska, who indicated he will seek to do away with the federal pay cap.

The increase would be aimed mainly at some 6,000 career managers in the elite Senior Executive Service, officials said, but the increase would go to a total of about 46,000 people. That total includes all of the 30,000 career and non-career executives from the rank of GS15 up whose pay has hit the $50,112 ceiling enforced by Congress since 1979. Also affected would be a new batch of upper level GS14s destined to hit the ceiling this year, plus Cabinet officers, members of the judiciary and others whose pay has been frozen at higher levels up to $69,630. The rise would not, officials emphasized, include members of Congress.

Congress has been reluctant to raise its own pay in recent years, and pay of senior executive branch officials is tied to the pay of congressmen.

The administration has already proposed a 4.8 percent increase for the rest of the federal white collar workforce.

Donald J. Devine, director of the federal Office of Personnel Management, told a Senate Governmental Affairs subcommittee hearing yesterday that the government is losing its best, most seasoned executives at a time when budget cuts and the resulting administrative problems make them more valuable than ever.

"But I believe the most alarming drain from SES is not the drain of talent or experience but the morale drain," he said. Although most federal executives say they like their work, surveys indicate that 52 percent are thinking about leaving the government because of inadequate pay, and "34 percent of all federal executives plan to look for a new job in the coming year."

"I believe the lack of any pay increase, even a small symbolic one, in the entire upper echelon of government will seriously aggravate an already intolerable situation," Devine said.

Rep. Vic Fazio (D-Calif.), vice chairman of the Federal Government Service Task Force, told the hearing that the private sector paid an average of $71,688, or 20 percent more than the Senior Executive Service ceiling for similar jobs.

Acting Treasury Secretary R. T. McNamar said his department supports the 4.8 percent increase. The Office of Management and Budget has advised him, he added, that it is in accordance with President Reagan's program.

Stevens, who chaired yesterday's hearings and has supported such a raise in the past, said, "Our task is to prove that it's costing the government more--in terms of real dollars--in training these people, replacement costs . . . It's costing our taxpayers more under the pay cap than it would cost if the cap were removed."

Sen. Thomas F. Eagleton of Missouri, the committee's ranking Democrat, said, "In the times in which we find ourselves, raising salaries of federal employes is not the most politically auspicious thing to recommend.

"I realize the mood of the country . . . when we talk about raising salaries for 'pointy-headed bureaucrats' and all that jargon . . . we're only kidding ourselves and we're only shortchanging our government when we pretend the problem doesn't exist and we continue to lose some of our most senior individuals."

The annual cost of the 4.8 percent raise would be about $90 million, Devine said. If the pay ceiling were lifted entirely, the price tag would be $270 million, according to a Stevens aide, and would mean a salary increase of as much as 20 percent for some top rank executives.

By law, congressional and other top federal salaries are supposed to increase automatically every year in step with the raises given to the general civil service workforce. However, in recent years, Congress has consistently used its appropriations authority to deny funds for raises at the top.