Four people were fatally shot and eight wounded in gang-style attacks across Southern California on Halloween and early today, and a 15-month-old boy died a day after he was shot in the head outside a birthday party. An unidentified 20-year-old man was killed early today while walking on a street in Monrovia, about 15 miles northeast of Los Angeles, sheriff's Deputy Detta Roberts said. A gang connection was being investigated; no arrests were made. In southwest Los Angeles, a 19-year-old man reputed to be a gang member was killed late Monday night when rival gang members ambushed a group of young people as they left a liquor store, officials said. Four people were wounded, one of them critically. The name of the man who was killed was not immediately released, and there were no arrests. In San Diego, two unidentified men were killed and three men wounded in three street shootings Monday night, authorities reported. Dalafayette Polk, the toddler injured in the birthday party shooting Sunday in the Watts section of Los Angeles, died at 12:30 p.m. Monday, said Paitoon Wisuskeow, a nurse at Martin Luther King Jr.-Drew Medical Center. Nine other people were wounded in the shooting Sunday night, one of the worst outbreaks of street violence by youth gangs. Police had made no arrests by early today. In downtown Los Angeles, a 15-year-old identified by police as a gang member was shot twice Monday night by by gang members driving by, said police Officer J.R. Smith. The teen-ager was reported in satisfactory condition at California Medical Center today.