Fire officials received reports of the crash at 7:53 pm local time (0253 GMT Thursday), and multiple agencies, including the US Coast Guard, were dispatched to the scene, the Los Angeles County Fire Department said in a statement, Xinhua reported.
"Two of the recovered victims have been transported to hospitals on the mainland, and two victims have been pronounced deceased at the scene," the fire department said.
Search and rescue operations are ongoing for a fifth victim who was reported in the aircraft, it added.
The US National Transportation Safety Board has been notified and will lead the investigation into the crash, the statement said.