Galloping out of the prestigious thickets of Foreign Affairs magazine, the Gang of Four has struck again. Their last ride was in pursuit of a U.S. policy of "no first use" of nuclear weapons. Given their identities (McGeorge Bundy, national security adviser to presidents Kennedy and Johnson; George F. Kennan, the celebrated Soviet expert; Robert S. McNamara, the former defense secretary; and Gerald Smith, a veteran arms control negotiator), it stirred a lot of dust, but no change in policy. The Gang members themselves were more than a little ambivalent.

Not so this time. With no ifs or buts, they have pretty nearly devastated President Reagan's cherished Strategic Defense Initiative (SDI). Better known as Star Wars, it promises nothing less than an airtight defense against nuclear weapons and an end forever to the threat of nuclear war. The president would make nuclear weapons "impotent and obsolete" by first developing a U.S. defense and then sharing the secrets with the Soviets.

With stunning unanimity, the Gang makes a compelling case that the proposal -- now far advanced, with a recommended $26 billion first installment for research and development, a timetable, and Ronald Reagan's name and prestige inextricably tied to it -- is a "dream" and an "act of folly" that "cannot be achieved."

There lies the real significance of the latest strike by the Gang of Four. They are not talking about an arguable element of nuclear strategy. Rather, they are attacking a fundamental and revolutionary transformation of nuclear policy with profound implications for the pursuit of nuclear arms control. And they are doing so with arguments readily available to Reagan in early 1983 -- if he had put the proposition to the same sort of rigorous test by responsible people on his own staff, in the bureaucracy, in the scientific community.

The awful truth -- the tellig commentary on his presidential style -- is that Reagan had no proposal worked out when he first floated the idea almost casually in a speech devoted to other, known quantities in his military program. He had only a fatuous, personal vision of a nuclear-free world.

The Gang of Four's denunciation of Star Wars is withering. To work at all, a nuclear-defense system would have to work perfectly: "A very few nuclear weapons, exploding on or near population centers, would be hideously too many." But "not one of Reagan's technical advisers claims that any such level of protection is attainable," including the officer in charge of the program. Thus, "the inescapable reality is that there is literally no hope that Star Wars can make nuclear weapons obsolete."

That being the case, by embarking on the effort with such fanfare the president is guaranteeing a Soviet counter-effort. The new result can only be a "large-scale expansion of both offensive and defensive systems on both sides."

More than enough authorities agree with this assessment to guarantee big trouble when Congress gets down to the business of voting on the huge sums of money the president is seeking -- the more so, given the enormous popular appeal of a nuclear-bomb-free world.

But the problem the president has created for himself only begins at home. The SDI is now inescapably an arms control issue with the Soviet Union, certain to be a top priority when Secretary of State George Shultz sits down with Andrei Gromyko next month to talk about how to proceed with the stalled nuclear arms negotiations. "The Soviets will want to stop SDI cold," says one administration arms control expert. But any effort to negotiate away projects still in the stage of research and development raises awesome problems of verification.

The more likely outcome, the Gang of Four and other authorities argue, is the breakdown of the existing antiballistic missile treaty of 1972 and a serious threat to progress on other arms control agreements with the Soviets. "In this real world it is preposterous to suppose that Star Wars can produce anything but the most determined Soviet effort to make it fruitless," the authors conclude. "The only kind of secret" that could be shared with the Soviet Union in the interest of making "each side durably invulnerable [is] one that exists only in Mr. Reagan's mind."

The Gang makes a plea to the president to abandon his dreams in the interest of a serious effort to achieve some progress on arms control in his second term. With a little more careful study, the foursome gently suggests, the president "will learn that it is possible to reach good agreements, or possible to insist on the Star Wars program as it stands, but wholly impossible to do both."

Failing that, the remedy must be found "in a long, hard, damage-limiting effort by Congress" guided by the words of the late Sen. Arthur Vandenberg when he found one or another proposition wanting: "The end is unattainable, the means harebrained, and the cost staggering."