U.S. Begins Exit from Thailand Bangkok Post 30th anniversary year Vol XXX no. 79         BANGKOK SUNDAY MARCH 21, 1976      BANGKOK, Thailand (AP) -- The United States began dismantling its bit radio listening post in northeast Thailand today following the Thai government's weekend decision that all but 270 of the 4,000 American military personnel in Thailand must leave the country within four months. Thousands of leftist students marched on the U.S. embassy Sunday for an anti-American demonstration.  A bomb thrown into the procession along the way killed four students and wounded more than 50.      Representatives of 18 rightist groups in favor of the U.S. military presence broke into the headquarters of the National Student Center of Thailand (NSTC), smashed furniture and broke windows.  They also demanded that Prime Minister Kukrit Pramoj "suppress Communists," investigate Soviet spying in Thailand and arrest a top leftist student leader.      Some NSCT leaders blamed the rightist groups for the bomb attack but police said they had no suspects.  One youth was badly beaten by students who believed he threw the bomb, but police let him go after questioning him.      A U.S. military spokesman said two C130 transports took 118 American personnel and equipment from the Ramasun Electronics Intelligence Center out of the country.      The listening post 300 miles northeast of Bangkok ceased its monitoring of radio transmission from Communist Indochina before midnight Saturday, the deadline set by the Thai government.  About 900 U.S. employees remained at Ramasun, a spokesman said, and the withdrawal of them and other American personnel is expected to continue at a rapid pace.