Secretary of State Cyrus R. Vance and Defense Secretary Harold Brown, noting that "while peace is expensive, war is more expensive," urged Congress yesterday to approve the administration's request for $4.8 billion to bolster the Egyptian-Israeli peace agreement.

In testimony before the Senate Foreign Relations Committee, the two Cabinet officials kicked off the administration's campaign for funds to honor the commitments made by President Carter to Israel and Egypt during the U.S.-mediated treaty negotiations.

Carter has called the proposed package of military and development aid measures the "most important foreigh affairs legislation currently before Congress."

But, while both Democratic and Republican members of the committee praised the administration lavishly for its efforts in bringing about the peace treaty, they also pointed to the need to explain to the American taxpayers why the United States should pay the costs of peace between the two longtime Middle East adversaries.

In response, Vance said: "Four wars in the Middle East have cost the U.S. taxpayers several tens of billions of dollars in direct costs alone. The cost of peace is modest when compared with the cost further war."

The State Department, he added, is working on a still-uncompleted analysis of the comparative differences in the costs that would be generated by situations of war or peace in the region.

Vance offered one example to back up his argument. He noted that replacing Israel's losses of military equipment during the October 1973 war had cost the United States $2.2 billion and that further U.S. military assistance to Israel since that time has amounted to approximately $5 billion.

With their decision to move toward peace, Vance asserted, "both Israel and Egypt are taking a step into the unknown. In order for both governments to lead their people through these uncharted waters, they must be confident that they can deal effectively with threats to their continued security."

But, he added, both countries face immediate and serious economic problems connected with the peace treaty-Israel because of the cost of its large-scale military withdrawal from the Sinai Peninsula, and Egpyt because of President Anwar Sadat's "urgent and critical need to demonstrate to its people the economic benefits of peace."

The administration's proposals for helping with these problems-a one-shot deal that is in addition to ongoing U.S. assistance to the two countries-involves $800 million to help Israel relocate two air bases from the Sinai and $2.2 billion in foreign military sales credits for Israel. Egypt would get $1.5 billion in military sales credits to replace outmoded equipment and $300 million in development aid.

Both Cabinet officials stressed several times that, although the proposals have a $4.8 billion price tag over a three-year period, the actual outlays to be made from the U.S. Treasury will total about $1.47 billion. That's because most of the requested funds involve military credit sales where Congress has to appropriate only 10 percent of the amount.

Asked whether a congressional refusal to support the aid requests would kill the peace agreement, Vance replied that it would "have every serious consequences" on the entire Middle East peace effort Brown added, "It would certainly undermine our influence to say the least."

Vance also called for congressional backing of Carter's commitment to extend to 15 years the five-year agreement the United States made in 1975 to supply oil to Israel in case of emergency.

He pointed out that Israel has had no need until now to invoke the agreement and said that even should the need arise in the future, Israel's requirements-about 165,000 barrels a day or less than 1 percent of U.S. consumption-would be "hardly noticeable to us."

Looking ahead to be next phase of the peace effort-negotiation of autonomy for the Palestinian inhabitants of the Israel-occupied West Bank and Gaza Strip-Vance conceded that so far there are no signs that Jordan and the Palestinians will cooperate in the effort.

But, he insisted, "as the negotiations progress and they see it's a reality that offers a chance to solve the crucial Palestinian question, it's our hope that they will join in the process, although I can't predict with absolute certainty that they will."

Asked about the action of most Arab countries in declaring an economic boycott against Egypt, Vance said the U.S. interpretation of the decision is that existing Arab aid commitments to Egypt-about $1 billion annually-will be honored but that no further aid will be granted.

But, he added, the actual positions of different Arb states like Saudi Arabia are not clear and "it remains to be seen how the boycott will be implemented." Again, he stressed, the outcome could be influenced heavily by whether the West Bank and Gaza negotiations make progress on the Palestinian issue acceptable to the Arab world. CAPTION: Picture 1, Secretaries Vance, left, and Brown back $4.9 billion for Egypt and Israel at Hill hearing; Picture 2, Sens. Jacob Javits (R.N.Y.) and Frank Church (D-Idaho) huddle. Photos by James K.W. Atherton-The Washington Post