The sheriff's department says it happened at the San Marco Boulevard exit near Bay Point. The shooting was on city streets close to Willow Pass Road.
The CHP said traffic on Highway 4 was backed up due to the shooting.
Claycord.com reported the shots came from a silver vehicle with a broken window. Investigators have not said if anyone has been hit.
On Saturday, one person was shot and killed on Highway 4 near San Marco Boulevard in Bay Point. A 20-year-old man was killed and investigators have not made any arrests in that case.
VIDEO: Pittsburg father killed in drive-by on Highway 4 mourned by coworkers