Deputies later discovered that the Hyundai had been reported stolen, and a loaded gun was found in the possession of one of the deceased, the sheriff's department said in a news release.
The vehicle, a 2018 Hyundai, collided with a 2010 Ford Mustang in an intersection in Upland, a city more than 33 miles (53.11 kilometers) east of downtown Los Angeles, according to the sheriff's department. The Hyundai then crashed into a pole, separated into three parts and caught fire.
The Hyundai's five male occupants were ejected from the vehicle, the sheriff's department said. Four of them were pronounced dead at the scene, and a 13-year-old boy was taken to the hospital. He is expected to survive.
The Mustang's occupants, a 35-year-old man and a 21-year-old woman, were taken to the hospital as a precaution. None of the occupants' names were released.
Other details were not immediately available.