It happened at about 4 pm Tuesday on McHenry Avenue, north of Hogue Road.
The California Highway Patrol says a 19-year-old woman from Merced was driving a Mitsubishi northbound on McHenry when the vehicle veered into oncoming traffic.
The car crashed head-on with a Toyota and also collided with two other cars.
Both the driver of the Mitsubishi and her 22-year-old passenger died at the scene.
A 60-year-old woman in the Toyota and her one-year-old passenger were taken to nearby hospitals, where they later died from their injuries.
No one else was hurt.
It's unclear whether drugs or alcohol were factors in this crash.