Four youths from Howard Beach, N. Y., go on trial today on charges of murder, attempted murder and manslaughter in the Dec. 20 attack by a dozen white youths on three blacks in which one was killed when he fled onto a freeway and was hit by a car. The blacks had walked to a pizzeria in the mainly white Queens community after their car broke down. The defendants are Jon Lester, 17, Scott Kern, 18, Thomas Pirone, 17, and Jason Ladone, 16. Seven other whites face lesser charges. Mayor Edward I. Koch called the attackers a "lynching party," and the racial aspects of the incident generated national publicity.