The shooting was reported at Libby Lake Park around 8:50 p.m., and officers were searching for the suspected gunmen, police said.
Several people ran from the scene and possibly drove away in a dark grey Toyota Tacoma with tinted windows, an Oceanside police lieutenant said.
Two teenagers were rushed by ambulance to Palomar Hospital, while the other two died in the park.
All of the victims were under the age of 18, according to the lieutenant. Their names were being withheld until their families could be notified. No other details were immediately released.
A teenage couple was killed in the park in May 2011. Four gang members were charged in that murder. Prosecutors said the killings were the result of a turf war over the park involving rival gangs. Neither of the victims in that shooting were involved in gangs, investigators said.