Heavy fighting erupted in the Christian Phalangist enclave in southern Lebanon today as Palestinian guerrillas entrenched in the crusader castle at Beaufort blanketed a small Christian Lebanese village with an artillery barrage, killing four persons and wounding six more. The casualities were the worst in southern Lebanon in more than a year.

Christian militias commanded by Maj. Saad Haddad responded with heavy shelling on Beaufort and the coastal city of Tyre, amidst expectations of some kind of involvement by Israeli armed forces if the fighting continues at the current pace. Haddad's forces are supported by Israel, which monitors the Lebanese situation carefully.

The artillery duels, which began shortly before dawn and continued through midday, shattered the relative calm that has prevailed in southern Lebanon for several weeks.

Southern Lebanon, however, has been tense during the on-and-off redeployment of Syrian troops from Beirut and other coastal regions in the north to positions along the eastern border. The withdrawal of the Syrian forces, which appeared to be suspended today amidst warnings by Lebanese officials that a resumption of the civil war was imminent, raised fears in Israel that the Palestine Liberation Organization would escalate attacks in the south.

Today's fighting apparently began when Palestinians unleashed a predawn barrage from Beaufort on Dir Mimas, a small Christian village near Qlaiaa, several miles north of the Israeli border.

Military officials said 101mm and 122mm cannon, mortars and heavy machine guns opened fire on Dir Mimas and Ramat Arnoun, farther north. Authorities said the attack began again when rescue teams attempted to go into the target area to evacuate the wounded.

Dir Mimas, with a population of 1,500, lies in a valley directly beneath the almost impenetrable Beaufort Castle, a heavily fortified position atop a steep hill approximately midway between the Mediterranean coast and the Syrian border. The shelling was the heaviest since 700 pounds of artillery shells were fired on Dir Mimas in a 24-hour period Oct. 20.

Guerillas also reportedly infiltrated the village before daybreak and planted explosives by two houses, killing one Lebanese man and a girl.

Haddad, whose Israeli-supported militia controls a buffer zone five miles deep and 60 miles along the border from the Mediterranean to the foothills of Mt. Hermon, issued an urgent appeal on Israel radio for support by the Israeli Army.

"We can't afford this kind of loss of our population," Haddad said. "I call upon the international conscience to intervene to stop the massacre, and I call especially on Prime Minister [Menachem] Begin to fullfill his promises to help us."

He was referring to a warning issued by Begin Thursday that Israel will "not be passive" if the PLO takes advantage of the Syrian peacekeeping force's move and attacks Christians.

When asked what he specifically expects from Israel, Haddad replied: "Military help -- what the prime minister promises. It is clear what we want."

Since the March 1978, invasion by the Israeli Army to the Litani river, Israeli combat units have crossed into southern Lebanon several times to attack PLO positions, charging that U.N. peacekeeping forces there have been ineffective in stopping infiltration of guerilla units into the region.