Fully four months after being shocked into reality by Israeli Prime Minister Menachem Begin, President Carter still hammers away at what he called Begin's "six no's" of Israeli peace policy, a presidential fixation that may now be the last resource of Egyptian President Anwar Sadat.

Carter's continuing allusions to the "six no's" he got from Begin during their deadlocked talks here last March prove that U.S. pressures on Israel are not about to soften. At a small White House dinner party for media luminaries July 26, the president again revealed his inner commitment to keep pressure on Israel by a lengthy monologue on Begin's no to Carter's questions about Israeli concessions.

For Sadat, that constitutes reassurance of Carter's support at a dangerous Mideast turning point. Sadat has just about run out the string on his dramatic peace initiative. Indeed, he now betrays signs of erraticism. It is by no means certain that Secretary of State Cyrus Vance will be able to lead him back to direct negotiations during his current rescue mission to Jerusalem and Cairo.

The peril of Sadat's position today is evident from new pressures on him from Saudi Arabia, his principal banker. Convinced that the Sadat-Begin peace talks are going nowhere, the Saudis fear political revenge against Sadat from Eyptian hard-liners and a possible radicalization of Egyptian politics in a post-Sadat era. That would inevitably lead to far wider radicalization, make a prophet out of Syrian President Hafez Assad and bring the Soviet Union back to the Middle East flaunting U.S. failure.

Thus Sadat, the springer of surprises, might spring another surprise in Vance's face: declare that his own peace campaign has "proved" both Israeli intransigence and American weakness in moving its Jerusalem ally, and go back to the trenches.

But in hard fact, despite their tortuous path, Egyptian-Israeli peace talks have not been standing still since Sadat went to Jerusalem last November. Even if Begin's bargaining process means negotiating each "grain of the desert sand," as Begin himself proclaims, the process has softened Begin's position even on some of the "six no's" that so angered Carter.

The most significant of those changes was signaled by Moshe Dayan, Israel's foreign minister, on July 24. For the first time he opened the door to future Arab sovereignty over the West Bank - the crux of Sadat's problem. Unless Sadat can get Israel's approval of language that ensures eventual Arab sovereignty there, a first-step Egyptian-Israeli settlement is out of the question.

Dayan's words in a formal statement to the Israeli parliament were highly ambiguous and highly qualified. But they represented an important advance from Begin's initial locked-door position on the West Bank: that Israel had a claim to perpetual sovereignty over the Arab-occupied West Bank because the Bible said so.

That position has been slowly crumbling, a process that Dayan accelerated when he declared that Israel "will be prepared" to discuss West Bank sovereignty after a five-year local autonomy period.

For Sadat, that is small West Bank pickings after eight months of negotiating. It dramatizes the "each grain of sand" negotiating tactic of the Israelis. It feeds his worry that, as one U.S. expert puts it, Begin intends "to nickel and dime him to death" at the bargaining table. It makes him an ever riper target for his critics and detractors in the Arab world.

Sadat's style is the opposite. He viewed his "sacred mission" to Jerusalem as a grand, irresistible gesture that would sweep away 30 years of the Arab-Israeli blood feud and bring swift accord. Not made for nickel-and-dime bargaining. Sadat has taken no public notice of Dayan's July 24 speech to the Israeli parliament.

That explains the hazards of Vance's trip to the Mideast snakepit. Vance is not ready to give Sadat what he has been pleading for: an American plan for the West Bank that perforce would put the United States hard on the Arab side and against Israel.

The Carter-Vance strategy calls for more movement by Israel before the United States goes out on a limb - one that Israel's political backers here would immediately try to cut off.

Sadat, sphinx-like, has given not the slightest hint what he will tell Vance. But if he agrees on another round of direct negotiations, the reason will be Jimmy Carter's courageous, unchanged commitment not to quit but to keep quiet pressure on Israel and watch the nickels and dimes pile up.