The continuing crisis in Iran has forced its way into a dominant position in the four-power talks that begin today in Guadeloupe, where President Carter is set to tell his most important European allies that he continues to support fully the shah of Iran.

That reassurance will be given against a background of an accelerating crumbling of Shah Mohammad Reza Pahlavi's hold on power in Iran and a rapidly widening suspicion among some U.S. officials that the shah's departure from Tehran yesterday for a quick trip to a resort in northern Iran may be the first step in his leaving within a matter of days.

Officially, State Department and White House aides said they had no information on the shah's plans. And it became more difficult for reporters to obtain unofficial comment yesterday after Undersecretary of State David D. Newsom told members of the Iranian task force he heads that he expects to be briefed on any background discussions they held with reporters.

At a meeting of the interagency group, Newsom reportedly emphasized that he expects its members to keep their recommendations on handling the Iranian crisis confidential. The main target of Newson's complaint appeared to be a detailed account in The Washington Post of policy divisions that surfaced at a task force meeting on Tuesday.

State Department spokesman Hodding Carter denied that the administration was attempting to gag dissenters. He noted, however, that "the constant leaking of advice is not useful to the policy process."

Originally conceived as a chance for Carter and the leaders of Britain, France and West Germany to talk informally about strategic and economic problems the four nations are liekly to face in the 1980s, the Guadeloupe conference now will have to take up two more immediate problems, the Iranian crisis and U.S. recognition this week of the People's Republic of China, U.S. officials acknowledged.

At a briefing at the White House Wednesday, reporters were told that in Guadeloupe President Carter will reiterate his support of the shah. The Associated Press, which was not present at the briefing, yesterday identified national security affairs adviser Zbigniew Brzezinski as the briefer.

U.S. officials said that Secretary of State Cyrus R. Vance -- who will not be in Guadeloupe -- has succeeded over the past 10 days in putting some new nuance into policy pronouncements as the shah's chances for holding power have continued to dim.

Previously, the pronouncements totally reflected Brzezinski's view that the administrtion had to be seen as supporting the shah without qualification. While still voicing support for the shah, State Department policy guidance notes now also emphasize American ties to Iran as a country.

The absence of State Department representatives in the small U.S. party accompanying Carter and Brzezinski could also result in a difference of accent put on American discussions of the decision to establish relations with Peking, U.S. officials said.

It is not clear whether the national security advisers the four leaders took with them will sit in on all of the sessions at the two-day conference.

U.S. officials expect West Germany to press for a more detailed explanation of the last-minute rush by Washington toward relations with Peking, which Brzezinski successfully managed. Bonn is thought to fear that American acquiesence in British and French arms sales to China could create new tensions for the western allies with the Soviet Union.

In a polite way, the Germans may be questioning Carter to find out if the China recognition is part of a long-term U.S. strategy for managing superpower relations, or an unstudied response to a Chinese strategy of "playing the American card" against the Russians.

Despite such questioning, American specialists think Carter and Chancellor Helmut Schmidt are now on much better terms. Schmidt, who was openly critical of Carter's leadership earlier, now wants to appear to be a strong junior partner and ally to Carter, American specialists think.

The four leaders are almost certain to explore the various forums that now exist for discussions with the Soviet Union on reducing strategic and conventional arms levels, and on cutting back forces in Europe.

French President Valery Giscard d'Estaing is expected to push his call for a conference on disarmament in Europe, which some administration officials feel could undercut the current Mutual and Balanced Force Reduction (MBFR) talks. The United States will be seeking a clearer idea of France's willingness to talk about changes in format and content of the conference.

But the Carter administration is reportedly set to resist any attempts at Guadeloupe to link the French idea for a disarmament conference, the MBFR talks and the planning for the next round of the strategic arms limitation trety (SALT) talks with the Soviet Union, which will be known as SALT III.

These items remain fairly academic at this stage, according to State Department officials, who predict that the four leaders will concentrate more on discussing what type of intermediate range missile should be stationed in Europe in the near future.