The Reagan administration continued to say little or nothing yesterday as Israeli forces rolled through Lebanon for the fourth day and the Israeli air force took out Syrian missiles in the Bekaa valley.

With President Reagan and Secretary of State Alexander M. Haig Jr. in Bonn, special U.S. emissary Philip C. Habib in Damascus and an increasingly worried team of American policy makers meeting in the White House and State Department, the physically divided government was reported to be in "a listening posture," unable to make long-range decisions about U.S. policy.

It was also, from the public record, unwilling to call forcefully for Israel to halt its drive. Late Tuesday the United States vetoed a United Nations Security Council resolution threatening sanctions against Israel if it did not withdraw immediately.

Haig and his State Department spokesman in Washington spoke in relatively mild terms yesterday of U.S. "concern" about the widening conflict while continuing to avoid criticism of Israel or its Middle East opponents.

The main thrust of U.S. policy, as explained by official sources, was a private diplomatic effort to obtain a cease-fire as the first step toward eventual Israeli withdrawal with international guarantees against further Palestinian attacks from Lebanon.

A cease-fire "could be fairly close," an informed official said. But U.S. intelligence was expecting Israel to seek to improve its position, especially to take control of the important Beirut-to-Damascus road, before agreeing to stop.

At a press conference yesterday Moshe Arens, Israel's ambassador to the United States, said his country's objective continued to be to remove Palestine Liberation Organization artillery and rockets from the range of Israeli towns and settlements.

He added: "If there are no more complications with the Syrians, I would think we are very close to achievement of the objective."

At the White House, the administration's senior crisis management committee, headed by Vice President Bush, met twice during the day to assess a torrent of intelligence and diplomatic assessments. After a briefing by the Central Intelligence Agency on some of the same information, members of the Senate Intelligence Committee said they feared a "much wider conflict" if present trends continued.

Among the most serious considerations were the following:

* The potential for a military showdown of expanding proportions between Syria and Israel. U.S. officials tended to discount reports of Israeli bombing raids near Damascus, but said a Syrian decision to send still more troops to Lebanon could cause Israel to bomb Syria. Such a chain of developments could bring escalation all around.

* The position of the Soviet Union, which is Syria's superpower sponsor and has broad interests in the Middle East. As of last evening, there were no signs of the Soviets rushing emergency supplies of weapons, ammunition or other aid to Syria, nor was there any indication of unusual Soviet troop movements, according to officials.

Nevertheless, Israel's destruction of Soviet-supplied surface-to-air missiles and advanced jet fighters was thought to place Moscow on the spot. Developments yesterday, therefore, seemed to increase the possibility of Soviet gestures or actions in support of Syria.

* The possibility of involvement by other Islamic countries. There were some indications that Iran's militant government, which has called for a united front against Israel, was preparing to send a substantial number of volunteers to Lebanon through Syria. While it is unlikely that such troops would affect the course of the war, such support could have important political impact in the Arab world.

Many Arab governments have no love for Syria despite their official statements of solidarity. There is doubt that most of them would go beyond rhetorical support in the event of an Israeli-Syrian war.

* The potential for new international diplomacy when the fighting has stopped. Discussions of this likely next stage have taken a back seat, for the moment, to the military developments and efforts to obtain a cease-fire.

In a statement issued in Bonn, Reagan appealed to "all countries to join in an international humanitarian effort" to help Lebanese victims of the conflict. He said that the United States will make available $5 million in emergency relief supplies immediately and that he was asking Congress to provide $20 million for additional assistance.

Also in Bonn, Washington Post Staff Writer Lou Cannon reported that Haig, asked if he planned to go to the Middle East soon, responded by saying, "Not at this time."

State Department spokesman Alan Romberg confirmed The Washington Post report that the would-be assassins of Israeli Ambassador to Britain Shlomo Argov were believed not to be affiliated with the PLO as Israel has claimed. Romberg said information supplied by the British linked the suspects to a dissident Arab group, Black June, that opposes the policies and leadership of the PLO.