President Reagan has postponed but not averted a threatened confrontation with Congress over spending bills he deems excessive, administration officials said today.

The White House announced that Reagan signed a $25.3 billion spending bill for the Department of Transportation and related agencies that was slightly over Reagan's request. But officials said three and possibly four other appropriations measures that Congress has not sent to the president are candidates for vetoes.

Earlier this year, when Congress spurned many of his budget priorities, Reagan vowed to reject "budget-busting" appropriations bills that exceeded his targets. It was expected that Reagan would challenge Congress to a veto confrontation this summer. Now, officials say, that is not likely to happen until later.

Reagan has signed the first four spending measures sent to him, plus a $6.95 billion supplemental appropriation. In several cases, including the transportation bill, House-Senate conferees trimmed the measures in anticipation of a presidential veto.

However, an administration official said today, "We're only in the fourth inning."

David A. Stockman, director of the Office of Management and Budget, has warned members of Congress that three pending bills are candidates for veto.

They are State-Commerce-Justice Department appropriations, Treasury-Post Office-general government and the Agriculture Department bill. A fourth candidate, the huge Labor-Health and Human Services appropriations, is also expected to meet White House resistance, but it has not emerged from committee.

The $25.3 billion transportation bill signed today was $196 million over Reagan's request, according to OMB spokesman Edwin L. Dale Jr.

However, the bill did not include a ceiling on operating subsidies to mass transit systems that the White House had sought. The bill included more money for mass transit programs and less for the Federal Aviation Administration than Reagan requested in January.

The bill also includes a moratorium, until Oct. 15, on regulations proposed by the Office of Personnel Management that would place more emphasis on merit for within-grade pay increases and reductions-in-force for federal employes.

Reagan, who is vacationing at his mountaintop ranch near here, also signed a bill extending temporary authority to the Student Loan Marketing Association to consolidate outstanding loans into government-backed loans. The consolidation has been a matter of controversy and Congress is expected to take it up again after the Labor Day recess. The extension runs to Nov. 1.

Even though he signed the bill, Reagan objected to a provision that nullifies recent Department of Education rules that he said would have helped target student aid funds more toward those who need them.