Four of the most prestigious think tanks in the United States, Britain, France and Germany have put their heads together and come up with an astonishing degree of unanimity on what ails the Western "alliance" -- and what to do about it.

The challenges set forth are the familiar ones: how to manage East-West relations, the value of detente, the utility of military force, the security of vital resources, Poland, the Arab-Israeli conflict and all the rest. But the focus is fresh. The first imperative is new machinery. Most intriguing is the idea of organizing ad hoc "principal country" groupings, varying in number and membership, to deal with particular trouble spots.

Thus, the report urges immediate creation of a first group -- composed of the United States, Britain, France, Germany and Japan -- to mount a continuing "watch" on the problem of the Persian Gulf.

The crucial, underlying premise is that existing mechanisms (NATO, the 10-member European Economic Community, the annual seven-nation economic summits) are no longer up to the job of concerting policy -- and mobilizing effective action -- in defense of the worldwide interests of the Western industrialized democracies.

So far, so good. For its hardheaded acceptance of just how much is missing, structurally, in the capacity of the West to consult and act effectively, the report ("Western Security: What Has Changed? What Should Be Done?") by the New York Council on Foreign Relations and its British, French and German opposite numbers deserves, well, two cheers.

A third cheer will be in order when we know better to what extent the problem is a matter of mechanics. "Procedures and machinery don't count," said a high-ranking French dignitary who passed through town the other day, one of many making the grand alliance pilgrimage to Ronald Reagan's Washington. "What counts is the willingness to use them."

On that score, right now, appearances deceive. For the new American administration, still in the process of "reviewing" and "studying" -- of making policy -- this is some kind of Happy Hour. With one or two notable exceptions, the hard questions can still be deftly set aside. Whatever's said, Heaven forbid that any offense is meant.

But once free and clear of the formalities -- in background briefings, luncheon gatherings and after-dinner speaking (not for attribution, please) -- the tiptoeing and temperature-taking gives way to frank talk. And what assorted representatives of our Western allies are saying raises profound questions about that "willingness" the Frenchmen mentioned to make good use of even the most promising new machinery.

The Haig/Reagan Crusade for Freedom for El Salvador, to start with, is at best a puzzlement. A British dignitary finds "our side" (the supposedly centrist junta) as "unsavory" as a leftist insurgency with Soviet/Cuban/Communist sponsorship. The elevation of El Salvador as the centerpeice of the East-West struggle strikes most Europeans as, in the words of one recent visitor, "an implausible test."

"The United States has no Third World policy," a West German complains. "You want to bilateralize [East-West] and we want to work behind the scenes," he adds, by way of explaining the efforts by West German Social Democrats to promote a negotiated settlement between the junta and the left. As for the administration's new human rights distinction between "authoritarian" and "totalitarian" governments, he finds it "specious."

A Frenchman scorns the utility of an American Rapid Deployment Force. Still less can Europeans think of a useful purpose for a permanent American military presence on the ground in the Middle East. Defending the developing European intervention in the Palestinian issue, one Briton acidly contrasts the United States' outrage over Afghanistan with its acquiescence to "14 years of Israeli occupation of Arab territory."

A countryman has a more candid explanation for the so-called European "initiative": The tilt to the Arab side of the argument, he will concede, has largely to do with oil.

What it all comes down to is a conflict of interest -- or self-interest. As one American puts it, gently, "There's a marked difference in public moods." With Europe, it's a matter of simple "proximity"; of economic stakes "acquired in 10 years of detente"; of a divided Germany's "special sensitivities."

The result is a deep difference in approach -- to the Polish threat, to deployment of "theater nuclear forces," to the Soviets across the board. The Europeans would negotiate to avoid rear mament; the United States would rearm to promote negotiation.

You get the impression, in short, that the NATO alliance has worked too well. In Europe, the Europeans feel secure. They don't want their detente disturbed by American insistence on confronting the Soviets all over the world. That's a generalization, true. But the impression is inescapable: The divergencies now working against allied collaboration may be too much for any new consultative mechanisms or machinery to reconcile.