Los Angeles police were seeking a motive for the attacks at the house in Northridge, in the San Fernando Valley.
Lt. Terri Brinkmeyer said a 911 call led police to the large house around 4:30 a.m. Cmdr. Andrew Smith said the bodies of two men and two women were found face-down on a side yard of the property.
Investigators were questioning several people who were on the property at the time of the shooting, but no one has been detained or arrested.
Los Angeles City Councilman Mitchell Englander, who represents the area, said the house was split into several units for rent. He said at least a dozen people lived in the house, which contained several bedrooms, kitchenettes and mattresses on the floor.
"It appears from the nature of things that this was an unlicensed boarding facility," Englander said from the crime scene. He noted that one bedroom was only accessible through a window.
The house was on street lined with houses, schools and places of worship.
"This is a very safe neighborhood. This type of thing doesn't normally happen here," Englander said.