TheState Highway Patrolsays the car was traveling north on U.S. Highway 301 near Enfield when it went off the road and ended up partially submerged in Beach Creek.
Troopers say the two women and two men in the vehicle had been in the water for at least 10 hours.
"They weren't buckled in. They may have tried to get out, but the pressure was too much for them to be able to get out," says state trooper K.D. Coggins.
The victims have been identified as Herber Loera, 16, of Smithfield; sisters Sonia Tellez, 18, and Jaquceline Tellez, 19, of Selma; and Jorge Munquia, 20, of Selma.
Authorities believe the driver may have lost control of the vehicle or fell asleep at the wheel.