The Senate yesterday postponed its scheduled consideration of the fiscal 1980 budget until Monday after Democratic leaders failed to resolve a dispute among committee chairmen over how to make good on $4 billion in promised spending cuts.

The decision came after Senate Democrats spent five fruitless hours behind closed doors, caucusing. Although Democratic leaders plan to caucus again today, the dispute could hold up the budget debate for days.

Meanwhile, Senate Budget Committee Chairman Edmund S. Muskie (D-Maine) said he will oppose on the floor President Carter's request to increase fiscal 1980 defense spending 3 percent after inflation.

Carter announced on Tuesday he would request the full 3 percent increase as a gesture to Senate conservatives, who are demanding a 5 percent rise as the price for supporting the strategic arms limitation treaty.

Ironically, Muskie's opposition ultimately could help the president, who is trying to appear as a middle-of-the-roader in the fray. House Budget Committee Chairman Robert N. Giaimo (D-Conn.) also opposes Carter's request.

The stalemate over the $4 billion in promised spending cuts underscored a major dilemma in Congress' consideration of the budget: Although almost every senator wants to see the deficit lowered, no one wants to cut his pet programs.

The dispute arose because when Congress set its initial fiscal 1980 spending targets last spring, it based its figures partly on the assumption that individual committees would enact legislation that resulted in "savings" from 1979 levels.

The $23 billion deficit level approved back then included calculations for $5.6 billion in reductions from current-year programs. However, the lawmakers actually have approved only a portion of these. Currently, they are some $4 billion behind.

The budget deficit already has grown to $28 billion in large part because of higher inflation and unemployment expenses. But if the "savings" issue is not resolved, it could rise to $32 billion -- too high for most lawmakers.

The difficulty is that paring back spending now would require revisions in money bills by several powerful committee chairmen, including Finance Committee chief Russell B. Long (D-La.) and Appropriations head Warren G. Magnuson (D-Wash.).

And both men indicated after yesterday's caucuses that they still were not prepared to roll back already enacted legislation -- at least not by the Sept. 25 deadline sought by the Budget Committee.

Long said yesterday that requiring his committee to review legislation the Senate already had passed would mean "asking people to reverse themselves" and might crimp state governments that had been counting on federal grants.

"It looks to me we'd be compelled to lay before the Senate something the Senate would vote down," he said. "We find ourselves in a trap looking for $1 billion in 'savings' that never was there."

And Magnuson told reporters he would insist on more time to finish work on his panel's 14 appropriations bills before considering any cutbacks. He said any attempt to hold the line now would squeeze major social programs.

Muskie suggested yesterday the lawmakers may decide to go ahead anyway and approve broad ceilings for spending and tax-receipts and then vote separately as each bill comes up on whether it wants to exceed those limits.

However, the Budget Committee chairman conceded that would give the advantage to the seven or so appropriations bills that already have been passed under the current limitations -- and touch off a stampede to push the rest through.

Among those still not enacted are the money bills for defense and the Interior Department. Technically, Congress is supposed to finish work on its appropriations bills before revising the budget resolution. But this year the lawmakers are late.

The battle over defense spending is expected to be a major controversy in the budget debate this time. Sen. Ernest F. Hollings (D-S.C.) is expected to offer an amendment for the hawks seeking a 3 percent rise this year and 5 percent in 1981.