The shooting occurred around 7:15 p.m. at Fairfield High School, near Schafer Stadium as the Fairfield-Suisun Unified School District was holding a commencement ceremony for Sem Yeto High School.
School officials confirmed the violence broke out just after the event ended, prompting police to declare a "large-scale incident."
The unidentified 18-year-old died at the scene.
Police have not confirmed whether the victim was a student.
Emergency responders transported the three surviving victims - ages 11, 20 and 25 - to area hospitals.
Their medical conditions are unknown at this time.
While investigators stated there is no ongoing threat to the community, no suspects are in custody and no arrests have been announced.