Fierce gun battles resumed today between Shiite Moslem and Druze militiamen in Moslem-controlled west Beirut, shattering a shaky truce and raising the toll in four days of fighting to at least 55 dead and 255 wounded.

The militias and their supporters used mortars, rockets and recoilless rifles to battle for control of neighborhoods. Efforts of Nabih Berri, leader of the Shiite militia Amal, to get his fighters to abide by the cease-fire went unheeded.

Forces of the Sunni Moslem Morabitoun militia fought alongside the Druze to suppress Amal, which has had growing influence in west Beirut, a region previously dominated by the Sunnis. Elements of the Syrian-sponsored Arab Democratic Party and the National Syrian Social Party also were seen firing on Amal lines.

Marwan Hamadeh, a Druze former Cabinet minister, visited Damascus today to confer with Syrian Vice President Abdul-Halim Khaddam on the situation. Parliament Speaker Hussein Husseini also went to Damascus for urgent consultations. Khaddam has been closely monitoring the latest conflict and spoke by telephone with Lebanese Prime Minister Rashid Karami, a Sunni Moslem, and Berri today.

The fighting began Wednesday night with a quarrel over the Lebanese flag. Druze gunmen tore down the red-white-and-green flag, replacing it with their party banner as preparations were under way for the celebration of Lebanese independence.

This morning, Amal militiamen appeared in the seafront Rouche district defiantly wearing tunics made of Lebanese flags. Combat raged around the Carlton Hotel, a Druze stronghold, and the Beaurivage Hotel in the southern outskirts of Beirut.

Meanwhile, Terry Waite, an Anglican church envoy seeking to negotiate the release of four Americans held by militant Moslem groups, remained in Beirut, unable to leave because of the fighting. When conditions permit, Waite is expected to go to the United States to meet with White House and State Department officials.