The teen-age son of a prominent Japanese legislator was killed and three companions were injured in an auto accident in Santa Maria, officials said. Kazunari Yamaguchi, 16, was the son of Toshio Yamaguchi, a former committee chairman in the lower house of the Japanese parliament, said a Japanese consular spokesman in Los Angeles. The younger Yamaguchi had been living with a host family in Beverly Hills while he attended school. He was driving on U.S. 101 about 150 miles northwest of Los Angeles when his car veered into the median and rolled over three or four times, Santa Barbara County sheriff's deputies said. The injured youths were Takanao Suganuma, 17, of Beverly Hills, who was in a hospital intensive care unit with head injuries; John Duncan, 16, of San Marino, hospitalized in stable condition, and Akiko Harano, 17, of La Habra, who suffered minor injuries.