Authorities said the pickup truck veered off the highway and struck a tree shortly before 3:30 a.m. just north of Mussel Shoals beach.
The victims were identified as Phet Chansy, 53, his 40-year-old wife Holly Hamm and their 11-year-old son Eric Chansy. The family was from the Long Beach area. All three were declared dead at the scene.
The fourth victim was identified as Monica Sara Sang, 45, a family friend from Long Beach. She was transported to Ventura County Medical Center, where she later died.
The Chansy's 13-year-old son walked away from the crash and was taken to the hospital with moderate injuries. Family members said he was badly bruised and suffered a broken wrist.
The cause is under investigation. However, autopsy results show Phet Chansy may have had a heart attack as he drove the extended-cab pickup that was towing an aluminum boat. According to the Ventura County Star, the autopsy found the driver had significant heart disease and evidence of previous heart attacks.
Family members said Phet Chansy loved to go fishing on his boat.
"It hurt me because he's a very nice guy," Chansy's brother-in-law said. "When he got fish, he gave me some fish sometimes. Now he's gone. It's unexpected."
The Associated Press contributed to this report.