A 14-year-old boy called police about 11pm on Saturday, local time, to report that his father had shot him and his brother at their condominium, said Sergeant Peter Rodriguez of the police department in Brea, which was investigating the shooting in neighbouring Yorba Linda.
Yorba Linda is a bedroom community in Orange County, 65 kilometres south-east of Los Angeles.
Police found the body of a woman on the doorstep. Inside, they found the bodies of a five-year-old boy and two girls, ages eight and nine. Officers also found the body of a man with a shotgun beneath him.
Police did not immediately know the motive for the shootings or the relationship of the victims.
The teenager who called police was hospitalised in stable condition, Rodriguez said.