Los Angeles city fire officials said a 55-year-old man was electrocuted after a tree downed a power line. Several people stranded near the Los Angeles River had to be rescued with inflatable boats.
Two other people died in car accidents in the San Diego area, and a fourth was died in a submerged vehicle, local media reported.
Another person was injured after her car fell into a massive sinkhole in Los Angeles, local television station KABC reported. She was trapped until fire crews pulled her out.
Rushing flood waters swept away cars, downed trees and caused mudslides and sinkholes, the National Weather Service said.
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Flash-flood warnings will continue through the weekend in many areas of the West Coast state, which has been hit this winter by a series of storms that have filled reservoirs, bringing respite following a severe five-year drought.
Although the latest storm, which packed heavy wind-driven rain, was mainly affecting southern and central California, rain was also forecast to hit the San Francisco Bay Area in the north.
"The storm looks to be the strongest to hit southwest California this season," the National Weather Service in Los Angeles said.
The National Weather Service said the northern part of the state -- where flooding last week damaged the Lake Oroville Dam and forced the evacuation of nearly 200,000 people -- was expected to see new rain and snow systems moving in during the next few days.
"Recent storms have left the region highly vulnerable, so amplified impacts will be possible with additional rainfall," the NWS said.