After four days of extraordinary diplomacy conducted largely behind the scenes, President Carter has turned a corner in his efforts to limit the damage of the split in the Arab world, retaining the U.S. option for an overall Arab-Israeli settlement.

Nothing is certain in the Arab Middle East. But the studied refusal of Syrian President Hafez Assad to link his future to Libya, Iraq and other redical Arab regimes holds the door open for a political breakthough going beyond a separate Egypt-Israel peace.

Secret plans were afoot late last week for Secretary of State Cyrus Vance to make a sudden, Dramatic shuttle to the Middle East. He would return here by Dec. 16 with probable stops in Saudi Arabia (angry at President Anwar Sadat's failure to check in before he went to Jerusalem); Damascus (where Assad's care in keeping his pro-U.S. options open have been duly noted in the White House); and possibly Jordan and Egypt.

Central to Carter's Mideast holding action is to prevent Assad from straying so far from the U.S.-backed Geneva course that he will be forced into the full - and dangerous - embrace of the Soviet Union. Indeed, that became the President's chief goal as soon as the extraordinary personal diplomacy of Egypt's Anwar Sadat began to unfold three weeks ago.

But the time needed to catch Assad's coattails before they disappeared into the Kremlin forced an embarrassing stall in Carter's Mideast policies. That stall cost him valuable political support at home and the private fury of Egyptian leaders in Cairo. But experts here believe the risk may pay off, preserving a corner of the Mideast stage for the United States to play is essential role despite the emotional drama of the Egyptian-Israeli duet.

During that embarrassing stall, U.S. diplomats were hammering home in Damascus, Riyadh and Amman the Carter appeal to stay hooked to the U.S. Geneva plan. Here in Washington, Carter was being reviled, accused of sulking for having been upstaged by Sadat and Israeli Prime Minister Menahem Begin.

But this anti-Carter mood, dominant among Jewish leaders and pro-Israeli politicians who felt the President was subtly undermining the Sadat-Begin duet, began to abate last last week. One reason: the propsect of widespread Soviet influence in the Arab world. However unwise the President had been to issue the joint Oct. 1 U.S.-Soviet Mideast statement that brought Moscow back into the center of Mideast diplomacy, the Soviet expectation arising out of a genuine Arab world split is mouth-watering by comparison, and frightens Israel and its friends.

Thus, diplomatic operatives here watched closely for omens from the Moscow journey of Assad's foreign minister, Abdul Halim Khaddam. There were several, and they were favorable. Not one official word was said about the Khaddam mission; no deals were cut.

Back in Damascus, where Henry Kissinger spent so much time cultivating Assad, not a single demonstration (as we write this) has been mounted against Sadat from the militant university student body. Indeed, the dominant Sunni Moslems (with the exception of Assad's own Alewite sect) was publicly backing Sadat's political bravery in daring to break down Arab-Jewish barricades of a lifetime. To top White House and State Department operatives, this means one thing: Assad, fearful of an irreconcilable Arab-world split is holding the kremlin at arm's length and hoping for reconciliation with his old friend Sadat.

But that is not all. Keeping its position confidential, Saudi Arabia, the pro-U.S. oil kingdom so terrified of radical political elements in the Arab world has informed the United States it definitely backs Sadat's celebrated diplomatic initiative - but will not say so publicly until all the Arab states bordering Israel also back Egypt. They are Syria, Jordan and Lebanon.

For Carter, the domestic political loss resulting from his soft-pedal acclaim for Sadat will fade away if he can consummate his present line and bring all the parties to Geneva. It is still a very big "if". However, considering the alternative - a lasting division in the Arab world and full re-entry of Moscow calling signals for the radicals - small progress is not to be ignored. Caught in Sadat's wake, Carter is suddenly beginning to move.