THE PRESS has been pleased to refer to itself as the "Fourth Branch" of the United States Government. If this were ever true, it is not now. Since late January 1977 the Fourth Branch has consisted of Doctor Henry A. Kissinger. For young adults, or old adults with short memories, it must be pointed out that this is quite unusual. It could even be regarded as phenomenal.

Consider Dean Acheson: Long after his tenure in the Truman cabinet, he was frequently sought out by both Congress and the White House for advice and opinion.But Acheson proferred his views in the capacity of an Elder Statesman. And Dean Rusk: Secretary of state for eight turbulent years in the Kennedy and Johnson administrations, Rusk has been infrequently solicited for and has only recently volunteered his counsel. But Rusk, a reticent man with few of the flamboyant qualities that make for "good copy," clearly prefers teaching to preaching. It has been left to Henry Kissinger to turn his old post in Foggy Bottom into a thriving third career. He continues to pass judgment on matters that will affect future generations; he influences important legislation; he treats as an equal with heads of state.

It is hard to keep up with Henry. In the autumn, he broods about human rights with the Trilateral Commission; in the winter he gives advice on SALT and Iran to The Economist of London. One week he has private conversations with Chancellor Schmidt of Germany; the next, he writes a column on the Middle East for The New York Times under the pseudonym "James Reston." In between, he advises the Chase Manhattan Bank and British General Electric on global economic problems. Until one remembers that he is the former secretary of state, one is tempted to tell Henry to stay in Foggy Bottom and mind the store.

EXCEPT FOR George Marshall -- who was a general with a handsome pension and simple tastes -- and Edward Stettinius and Christian Herter -- who held the job only briefly -- Henry Kissinger has been the only nonlawyer to have presided over the Department of State since Henry Stimson a half-century ago. As is well known, lawyers who serve the government always have an El Dorado awaiting them; it is called A Frim. But professors? After having been kissed by Anwar Sadat, Golda Meir and Liv Ullman, teaching Foreign Affairs II would be thin gruel. For anyone, let alone Prof. Kissinger, there is an unbridgeable distance between "Here Comes Superman!" and "Goodbye, Mr. Chips." Henry clearly had no choice but to make the position of Former Secretary of State a full-time job.

But it is one thing to aspire, another to achieve. Kissinger was, after all, a member of an administration that met with ineffable disgrace. Yet he has been able to keep from drowning in anonymity. According to Balthasar Gracian, the canny 17th-century Jesuit, "Few in life are felt to deserve an encore. Fortune rarely accompanies any one to the door: warmly as she may welcome the coming, she speeds but coldly the parting guest." Gracian had it right -- for most people -- politicians, statesmen and bureaucrats alike. But not for Henry A. Kissinger! He is still giving encores more than two years after the curtain fell on his Broadway run.

One may regard Kissinger as "The Greatest Secretary of State in America's History," as breathless Kissinger groupies in Washington and New York have been heard to say Or, more coolly, he may be viewed as having been smart, competent and energetic -- as good as any and better than most. But the fact remains that Kissinger's views are still regarded as being not merely informative as, say, Former Secretary Dean Rusk's are, but as influential , even decisive .

How has the Former Secretary managed to remain squarely athwart the foreign affairs policy-making process?For openers, he is definitely not your usual Washington in-and-outer -- entering his oak-paneled office on Jan. 21 excited and exciting, sitting there on a lovely Sunday afternoon depressed and depressing, closing the door behind him a few years later, a decade older.

Henry Kissinger may turn out to be the Stuff of History; he certainly is the Stuff of Theater. Hero of a best-selling novel (touted by him as a "great work of fiction"), Kissinger is a Personality; he may even be a Character -- an intellectual, with a good nose for buried treasure; a charter member of the bourgeoisie with a strong sense of noblesse oblige ; an arrogant, ambitious, amusing, secretive, sensitive, serious, bright, brash, broody man.

Fair enough -- some of the best of my ambitious, sensitive, bourgeois friends have some best friends who are bright, arrogant and secretive. What makes the Rich and Great seek out Henry? Why are his pronouncements on the most trivial or profound matters of state regarded as being worth $10,000 a throw, or as front-page news, or, most important, as a bell-wether of congressional support or rejection of a Carter initiative?

Perhaps it can all be explained by the brilliant foreign policy successes the United States enjoyed between 1969 and 1977. But before we weigh up the heavy global burdens Americans shed and assumed during Kissinger's stewardship, let us acknowledge two fundamental propositions: The first is that American diplomatic history did not start when the professor marched into Washington in early 1969. Nor did it end when he marched out eight years later. He inherited problems as well as opportunities from Dean Rusk; he left both ugly and comely legacies to Cyrus Vance.

And, then, an President Carter is fond of reminding us, much that happens in the world (for good as well as ill, Carter sometimes forgets to mention) is beyond the reach of American initiative or influence. It may be good politics for the Outs or the Ins to place blame or claim credit, but the Great Bookkeeper on High -- and even some politician-weary cynics here on earth -- can distinguish rhetoric from reality.

The Nixon administration inherited a mess in Asia, a resentful Latin America, a restless Africa, an inward-turning Western Europe, an angry Canada, an explosive Middle East. Kissinger can, and does, take credit for a degree of rapport with Moscow, the opening to China, SALT I, and some movement toward a modus vivendi between Israel and Egypt. But the Cyprus and the Greece-Turkey problems were passed along; so was a new mess in Indochina; so was a resentful Japan; and a troubling Soviet and Cuban presence in Africa; and a supine American posture with regard to the oil-rich nations of the Middle East; and a Latin America awash in Washington's indifference; and a Western Europe concerned by America's go-it-alone Soviet policy. Former Secretary of State Kissinger may have left the blobe better than he found it, but this is not altogether selfevident.

Much of his world, to be sure, was pushed and pulled by forces outside American control -- so was Acheson's, Dulles' and Rusk's. So is Vance's.

Much of the time, Washington -- just as Moscow or London or Cairo -- has had to react to or remain quiescent in the face of some other government's act of foresight or folly. Frequent "dramatic initiatives" and "carefully orchestrated foreign policies" are the makings of political oratory and Walter Mitty daydreams rather than of the real world confronted by responsible leaders. That is just as well -- manic national demonstrations of machismo have scarred every corner of the globe. And the percussion all too often drowns out the strings.

So not even Dr. Kissinger can have it both ways: He can accept the blame for Cambodia's tortured history since 1970, or he can acknowledge that, as in the case of Iran, events frequently outrun the best-laid plans of the most brilliant White House advisers. Indeed, as he must know as well as anyone, one of the most useful attributes of an American secretary of state in the latter part of the 20th century may be a talent for damage limitation.

ALL THIS is not to say that future scholars -- insulated by time from the effects of charisma, flackery and petty jealousies -- will not chalk up Dr. Kissinger as a good secretary -- perhaps even among the Top Ten.

But whether he will receive high marks as a Former Secretary is still an open question. Surely, the high quotient of low politics he exhibited in his recent Economist criticism of President Carter's feckless support of the shah of Iran will not add much to Kissinger's score. (Nor, for that matter, will the personalized and politicized level of George Ball's Economist critique of Kissinger add to the former undersecretary's.) One could fairly ask, for example, just what Kissinger would have done if his ambassador to Tehran, the former director of the CIA, had been smart enough to have detected simmering tension and smoldering anti-shah sentiment. The odds are high that Washington's posture -- considering the perceived stake the United States had in the shah -- would have been to "hang tough." And to pour in even more military equipment.

In another Economist interview, Kissinger shared his views on the arms limitation treaty (SALT), now in the final stage of negotiation with the Russians. His ultimate approval or disapproval of the treaty will probably make the difference between Senate ratification or rejection. The point is not lost on Dr. Kissinger. After all, Carter relied heavily on his endorsement to sell his Panama and Chile policies to Congress.

What is "your attitude to ratification [of SALT] by the Senate?" was the interviewer's first question. "We [note the royal 'We'] do not have a text to study and analyze," Kissinger demurred. But for the next several pages the Formed Secretary outlined, sometimes from Delphic hights, sometimes from ground zero, the kind of treaty he would wish to see before he will advise Senate approval.

Unlike his views on Iran, the SALT interview bespeaks a great deal of preparation and careful thought; Kissinger had earlier spent several hours with Paul Nitze, the earnest and influential critic of SALT. Not surprisingly, then, the Former Secretary finds some serious flaws in the draft treaty. Jimmy Carter will have his work cut out for him on Capitol Hill.

Aside from the obvious difference between having or not having responsibility for advice offered, there is another juicy advantage in being a former rather than a sitting secretary of state: Kissinger can pick and choose his issues; Vance must cope with all that come his way. Henry knows this, of course. It is tribute both to his sense of theater and -- it must be said -- fairness that he rations out his opinions to select (usually paying) audiences rather then spouting off to the great unwashed every hour on the hour.

However hard one works at it, however assiduously one courts and is courted by the pundits and the flacks, the role of a Former Something has a short half-life. Surely, there is a consequential Something Else in the Kissinger game plan. What else? The presidency in 1980 is out; Kissinger was born abroad. The Senate, perhaps? Let Gracian have the last word: "Even the phoenix uses its retirement for new adornment and turns absence into desire."