The stabbing occurred inside The Continental Room near Santa Fe Avenue and Harbor Boulevard just before 1:30 a.m.
Police say three men and one woman were transported to local hospitals with non-life-threatening stab wounds. All were expected to survive.
The motive for the stabbing remains unknown. No arrests have been made in the case.
Detectives planned to interview witnesses and review surveillance cameras from inside the bar and nearby businesses for possible leads in the case.
Anyone with information regarding the stabbing was urged to contact Fullerton police at (714) 738-6715.
CNS contributed to this report.