Lebanese Christian militia leader Maj. Saad Haddad, presiding over what for the moment is a relatively quiet Israeli-supported portion of southern Lebanon, pledged today that his artillery will remain silent until his enclave is shelled again by Palestinian guerrilla forces.

Haddad warned, however, that Palestinian commandos operating in areas controlled by U.N. peacekeeping forces have been taking advantage of the four-day lull in fighting, entrenching themselves and bringing in new weapons.

Haddad said two Soviet ships docked at the Lebanese port city of Tyre early this week and began unloading large stores of arms for Palestinian forces, including Katyusha rockets, bazookas and heavy mortars and that his Phalangist forces were braced for renewed attacks.

Nonetheless, Haddad's pledge not to initiate new artillery duels -- coupled with a suggestion today by Israeli Defense Minister Ezer Weizman that if southern Lebanon remains quiet, Israel may decide that it can suspend artillery bombardments and aerial attacks -- gave rise to hope for a prolonged cease-fire in the area.

Haddad's forces, which long have been supplied by Israel and which serve, in effect, as Israel's surrogate army in maintaining a cordon sanitaire against cross-border attacks by Palestinian guerrillas, are believed to number between 1,000 and 1,500 men.

For the fourth consecutive day, southern Lebanon remained relatively quiet today as three busloads of foreign reporters, on a trip sponsored by the Israeli Army, crossed Haddad's 70-mile-long "Republic of Free Lebanon" -- as Haddad has proclaimed the portion of Lebanon that his forces control. The tour apparently was intended to offset worldwide media attention to heavy Israeli shelling of civilian areas that are said to be laced with Palestinian commandos.

But the public relations effort seemed to fall short of expectations, if the Israelis intended to call attention to the effects of Palestinian shelling. The villages along the route showed little evidence of destruction apart from that incurred during the March 1978 retaliatory invasion of southern Lebanon following a terrorist attack along Israel's Tel Aviv highway.

There was ample evidence, however, to support Haddad's claims that his Israeli-supported forces arein control of the 400-square-mile enclave and that they enjoy the support of the estimated 80,000 inhabitants there.

As a few of Haddad's Sherman tanks, armored cars and truck-drawn howitzers rolled through this Maronite Christian town of 3,000 persons today in a memorial day parade for the town's civil war victims, townspeople lined the streets to cheer the militias and their flamboyant leader, Haddad.

Similarly, in Bint Jbail, barely a mile further east along Haddad's narrow corridor, the 70 touring reporters and their Israeli escorts received a warm welcome. As if expecting the motorcade, merchants lined the streets selling ciragettes and bottles of Scotch for a third of the price they bring in Israel.

Israeli currency was accepted freely, and the bright red Israeli buses with Hebrew marking seemed as at place in the village as the donkey carts and cars with Lebanese license tags.

Many of the residents who travel daily into Israel to work speak fluent Hebrew.

But aside from the widespread devastation left from last year's Israeli invasion, there was no sign of recent shelling by Palestinian guerillas, and townspeople reported that the area has been quiet for 14 months.

An Israeli liaison officer to Haddad's militias, however, said the stopover was intended to demonstrate to the reporters "the prosperity you see everywhere. Today is market day, and look at the crowds."

Many of the town's 5,000 residents, left from a population of 12,000 before the civil war, cheered the motorcade as it passed out of the village.

At Meiss al Jabal, a predominantly Shiite Moslem village, the Israeli escort officers pointed out a small shop that received a direct hit from an artillery shell Saturday, killing four members of a family. They said it was a Palestinian shell.

But it was the only town along the route at which recent casualties attributed to terrorist shelling were cited. The last village along the route, several miles outside Marjayoun, turned out to feature a planned pro-Haddad radio station, to be called "Voice of Hope" and backed by American pop singer Pat Boone and his "Church of High Adventure."

The radio station, housed in a stone building practically in the shadow of the PLO-controlled Christian crusader Beaufort Castle, showed signs of recent shelling. Haddad reportedly has told the radio station managers that when shelling starts, he wants them to broadcast the explosions to demonstrate the plight of the Christians.

Inexplicably, the Israeli-arranged tour omitted stops at Marjayoun and Qlaiaa, both of which have sustained heavy Palestinian artillery damage and which presumably would strengthen Israel's argument for retaliatory fire. Officials explained that the tour was behind schedule and that time was needed for camera crews to transport film back to Tel Aviv.

In the meantime, Haddad insisted that the U.N. Security Council, which is considering the Lebanon situation, "has a real comedy going," which could result in the "extermination" of the Maronite population in Lebanon.

"Now, it has been quiet for four days. But if they shell on us, we will answer," Haddad declared.

An Israeli liaison officer, contemplating the future of the enclave, said "Any prediction is for the prophets, and I'm not a prophet."