New York city's major commercial banks provisionally agreed yesterday to buy $500 million in city bonds over the next four years, making them the first of the participants in the proposed $4.5 billion financing package to make a commitment.

But Treasury Secretary W. Michael Blumenthal in a call to Municipal Assistance Corp. Chairman Felix Rohatyn, said the bank pledge was insufficient. And a Treasury Department spokesman indicated that the Carter administration, which is backing legislation to guarantee $2 billion in city bonds, expects the banks to finance "a significant amount" over $500 million to make the total package work.

Rohatyn, after a meeting of the MAC board, said the bank's commitment was conditioned on securing the federal loan guarantees, passage of a bill by the state legislature to extend the life of the city's fiscal monitor, the Emergency Financial Control Board, and a resolution of New York City's seasonal or short-term financing problems.

But apart from the agreement reached with the banks, the remaining pieces in the complex jigsaw of labor, political and financial negotiations that are taking place remain unresolved.

"I'm afraid it's in a lot of people's bones that they need to go down to the wire in the negotiations," commented one city official.

A Saturday deadline looms for completion of the financing package, based on the commitment city and state officials made in a meeting last week with Blumenthal. And that in turn requires settlement of two-year labor contracts, at least in principal, with 55 city unions, and passage of the control board legislation in Albany as well as a bill to extend the borrowing power of MAC.

Senate Banking Committee Chairman William Proxmire (D-Wis.) last week agreed to schedule hearings beginning May 24, provided all parties have made their commitments.

Proxmire already has indicated opposition to any further federal assistance to New York City, claiming that the city's banks and the municipal pensions funds have the wherewithal to see the city through the next four years without help from Washington.

Legislation has already passed the House Banking Committee, but a vote by the full House has not been scheduled yet.

On the labor front, Basil Patterson, the deputy mayor who is spearheading the municipal union talks, said the negotiations were "extremely tough" but said some progress was being made, "and what I'm aiming to do is have something" by today.

Gov. Hugh Carey had requested that the union issues be settled by today so that several days can be left for nailing down the financing package.

"We are encouraged," commented Rohatyn on the bank's commitment. "At least this is a first step. We see a bit more clearly all pieces on the table. Even if they don't fit, at least we can see a pattern."

Mayor Edward Koch, while calling the development "good news" also noted all of the other pieces that still must fall into place if New York City is to avert bankruptcy.

The current federal loan program to New York expires June 30. And without some new program, city officials are predicting that the city will run of money early this summer.