A report by a four-man private study group has concluded that hopes for a negotiated end to the Arab-Israeli conflict are fading and can be kept alive only if the United States broadens and intensifies its role as an impartial peacemaker in the region.

The report, made public yesterday, was written by Philip M. Klutznick, secretary of commerce under President Carter and a prominent American Jewish leader; Harold H. Saunders, assistant secretary of state for Mideast affairs in the Carter administration; Merle Thorpe Jr., a Washington attorney and president of the Foundation for Middle East Peace, and Joseph N. Greene Jr., president of the Seven Springs Center, a private scholarly organization in Mount Kisco, N.Y.

The four, who recently made an extensive Mideast tour, concluded: "Hopes for a negotiated peace are fading just at a moment when acceptance of Palestinian national identity in the Arab world and beyond and growing Arab willingness to accept the Israeli state have created the best possibility of an Arab-Palestinian-Israeli negotiation since Israel was established."

Their report also argued that "no peace will be possible without the Palestine Liberation Organization being involved in the process in some way" and said the United States should not rule out possible creation of an independent Palestinian state in Israeli-occupied territory as part of a settlement that includes Arab recognition of Israel's right to exist.

In addition, the four said they had found widespread agreement in the Middle East that only the United States "can effectively help to achieve peace," but they added "there is deep doubt" about whether the Reagan administration is willing to put as much emphasis on the peace process as it has on strengthening U.S. military capabilities in the area.

Calling on the United States "to wed military and diplomatic strength in a coherent strategy," the report suggested going beyond the Camp David process, which involves only negotiations between Israel and Egypt, and mounting "a parallel campaign" to involve such other Arab states as Saudi Arabia, Jordan and Syria in the peace talks with Israel.

The general thrust of the report is likely to draw criticism, particularly from Israel and its supporters in this country, because it supports ideas such as dealing with the PLO, considering the establishment of a Palestinian state and seeking a larger role for Saudi Arabia that are vehemently opposed by the Israeli government.