Trooper Guy Gill says a driver of a pickup truck was heading southbound Tuesday morning when he struck another vehicle and then hit the concrete jersey barrier.
Debris from the concrete jersey barrier went into the northbound lanes of I-5, and three vehicles heading in that direction struck it.
The crash near 38th Street caused major traffic backups in both directions.
Gill says none of the injuries were life-threatening. He says all four people who were hurt are alert and conscious.
Investigators are looking into whether the pickup driver's medical issues played a role in the collision.