President Reagan said yesterday that the $4.3 billion jobs program he proposed to the Democratic leadership in Congress would not be a major addition to the federal budget because most of the funds are already budgeted.

"There may be some" cost, Reagan told reporters in the Oval Office while posing for photographs with Vice President Bush and Secretary of State George P. Shultz, "but basically it is an acceleration of things that are already in the budget, moving them up."

Reagan's plan would advance the starting dates for federal construction projects, including road and bridge construction, and offer aid to the jobless, including extending unemployment benefits, opening government facilities to the homeless and providing food for the hungry.

An undisclosed amount of money would be provided for new federal projects to help reduce the nation's 10.4 percent unemployment rate.

In December the unemployment rate reached 10.8 percent, a post-Depression record, prompting Republicans and Democrats alike to pressure the White House for a jobs bill.

At that time the president condemned a $5.4 billion jobs bill as a "pork-barrel" proposal that would slow the nation's economic recovery by adding to the size of the deficit.

Democrats and Republicans persisted in demanding a jobs program, with House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) discussing proposals ranging from $5 billion to $10 billion.

Reagan's compromise proposal was presented to the Democrats Thursday.

White House spokesman Larry Speakes said yesterday that final details of the plan have yet to be agreed on.

He said the president had not anticipated that the first White House proposal would be accepted as a compromise.

"We had been in discussions with them," Speakes said, "and it was not anticipated that we would move this quickly."

House Majority Leader James C. Wright Jr. (D-Tex.) called Reagan's proposal an "excellent beginning," but cautioned that Democrats should consider it only the "first phase" of efforts to reduce unemployment.

Reagan also said he would nominate White House staffer Richard S. Williamson as the first U.S. ambassador to the International Atomic Energy Agency and other international organizations in Vienna.

William's nomination is subject to Senate confirmation.

He is currently assistant to the president for intergovernmental affairs.