Four conservative Republican senators have hired a former Defense Department aide fired by the Reagan administration for allegedly disclosing classified information to the media.

Michael E. Pillsbury, dismissed in April as assistant undersecretary of defense for policy planning, said yesterday that he has been hired to work on foreign policy and trade issues by Sens. Gordon J. Humphrey (N.H.), Chic Hecht (Nev.), Jesse Helms (N.C.) and Orrin G. Hatch (Utah).

The senators' hiring of Pillsbury reportedly angered Secretary of State George P. Shultz, who recently complained to congressional leaders that an alleged "leaker" should not be rewarded with a new job, according to a Senate aide. Some administration officials are known to be concerned because Hatch and Hecht are members of the Select Committee on Intelligence and have access to top-secret information.

But the senators or their aides defended the hiring, saying Pillsbury is a qualified foreign policy adviser and has not been convicted of any offense. Humphrey argued that Pillsbury was fired because he disagreed with other administration officials on key issues, not because he leaked information.

Underlying the dispute is the complaint of some conservative members of Congress that the administration is not providing enough aid to guerrilla movements fighting communist governments in Afghanistan, Angola and Kampuchea (Cambodia). In particular, Humphrey said the administration has not given Afghan rebels enough help, a cause that Pillsbury championed.

Humphrey "regards Michael Pillsbury as the most knowledgeable person in Washington regarding the administration's managerial neglect of the Afghanistan situation," said an aide to the senator. "He regards Pillsbury's dismissal as retaliation for Pillsbury's intruding on the turf of those in the administration who are guilty of this managerial neglect."

Administration officials who asked not to be identified said when Pillsbury was fired that he leaked information to The Washington Post about a decision to provide Stinger antiaircraft missiles to Angolan rebels. Pillsbury, a former Senate aide who maintained close relations with conservative senators during his Defense Department employment, has said he was not the source of the leak.

Shultz was traveling in Asia yesterday and could not be reached for comment. A spokesman for the Justice Department, which was reported to have opened a criminal investigation of the leak, would not comment yesterday.

Each of the four senators has donated $12,000 a year to hire Pillsbury, who is working in Humphrey's office. Hecht said he hired Pillsbury to do research on foreign policy and energy issues.

"I never second-guess the Defense Department. That was their decision," Hecht said. "But he's a very bright man, he's got a background with the Rand Corp., and I've hired him temporarily to work on these issues."

Hecht and a spokesman for Hatch said Pillsbury will not have access to classified information. Hatch's aide stressed that it has not been proved that Pillsbury did anything improper.

"The jury's still out on that one," the aide said. "He's been fired, but I think the investigation is ongoing . . . . We just thought we could make use of his expertise and his skills."