Teamsters' President Frank Fitzsimmons and three others have agreed to resign as trustees of the union's major pension fund and relinquish union control over its $1.4 billion in assets, the Labor Department and Internal Revenue Service announced yesterday.

The government-negotiated agreement, reached under the gun of threats to haul the trustees into court, calls for transferring control over the fund's vast real estate and other assets to independent investment managers, along with other changes aimed at correcting alleged improprieties.

In return, the IRS will restore the fund's tax exemption, which has been extended only on a temporary basis since it was revoked last June, according to the Labor-IRS statement.

The agreement culminates an 18-month government probe into charges of mismanagement, corruption and ties to organized crime on the part of the Teasmters' Chicago-based Central States, Southeast and Southwest Area Pension Fund, which provides retirement coverage for about 450,000 trucking employees in 33 states and is one of the nation's largest private pension funds.

The pact resulted from a get-tough stance by the Carter administration, particularly Labor Secretary Ray Marshall, who assumed personal command of the probe shortly after he took office in January amid rising congressional complaints about the pace of the inquiry under the Ford administration. A hearing on the investigation is scheduled today by the House Ways and Means Oversight Subcommittee.

Subcommittee Chairman J. J. Pickle (D-Tex.) was quoted by the Associated Press as saying the resignations were "very definitely a step in the right direction" but adding, "I personally will be interested in knowing who will be the new trustees and where is the control."

Marshall, who threatened to go to court to force the ouster of Fitzsimmons and the other three trustees if they didn't step down voluntarily, said litigation now appears unnecessary but did not rule it out if there is foot-dragging in carrying out the agreement.

"It now appears possible that we can avoid litigation," said Marshall in a separate statement heralding the agreement, "assuming that the fund trustees make good progress in following up on their commitments."

Legal action against any individuals suspected of mishandling funds will proceed, however, government attorneys said.

In what is apparently a face-saving gesture toward Fitzsimmons and his colleagues, the pact does not call for them to leave office until after the procedures for transferring control over the assets have been set in motion and the fund's tax exemption has been restored.

But it envisions speedy action. "The government contemplates that this (the prerequisites for resignation) will occur not later than April 30, 1977," the joint statement said.

Resigning as pension trustees along with Fitzsimmons will be another union representative, Teamsters Vice President Roy L. Williams, and two management representatives, John F. Spickerman Sr. and A. G. Massa - the only four trustees left after the fund removed 12 of its 16 trustees under government pressure last October.

It will be up to the fund to decide whether to replace the four or leave the positions vacant, according to a Labor Department lawyer.

When Marshall first demanded the resignations last month, the trustees balked, offering instead to transfer control of the fund's assets to outside sources. But Marshall continued to insist on clearing the deck of all hold-over trustees, announcing that "remedial action" - meaning court action - would be taken.

The cumulative trustee concessions, including forfeiture of control over the funds's assets, is the most far-reaching action taken in implementing the 1976 Employee Retirement Income Security Act, which the Labor Department administers, according to government attorneys.

Under the pact, hammered out in Washington over the weekend by the trustees in consultation with government lawyers, the trustees will choose the investment managers from among mortgage banking firms and banks of "recognized national stature," subject to veto to of their choice by the Labor Department.

The managers will recommend investment objectives for the fund in addition to assuming "responsibility for complete control of the fund's portfolio," more than two-thirds of which is in real estate, including Las Vegas casinos cemeteries, hospitals, union halls and other properties.

If the trustees do not reach "agreement in principle" on a choice acceptable to the department by April 10, an "interim committee" will be designated jointly by the fund and the department to expedite the process.

The fund also agreed to an independent review of loans and related transactions dating back to 1965, annual publication of a full financial statement, computerized record-keeping, establishment to the trustees and other procedures aimed at complying with federal laws, according to the Labor-IRS statement.

The tax exemption will be granted retroactive to Jan. 1, 1976, although negotiate over the status of the exemption from 1965 to 1976.