Khmer Rouge guerrillas have moved up to 4,000 refugees closer to the heavily shelled Cambodian frontier, evidently to stop relief agencies in Thailand from gaining greater access to them, sources said Wednesday. During the first week of May, the communists loaded the refugees onto Chinese-made military trucks along with their pigs and entire households in front of relief officials, the sources said. The 2,000 people left behind were mostly infirm or handicapped. The guerrillas evidently fear they will lose control over the refugees, whom the United Nations had planned to move to a new camp where they could get medical attention denied at Ta Luan.