Officials at the California Men's Colony, just outside of San Luis Obispo, are investigating a riot that occurred Wednesday afternoon.
The riot reportedly involved approximately 40 inmates in a medium custody yard.
It happened just before 1:15 p.m.
Officials say a group of approximately 20 inmates rushed a group of 10 inmates and another group of 10 inmates joined in the attack.
Officers reportedly used pepper spray and other non-lethal projectiles to stop the fighting.
CMC officials say at least one "inmate-manufactured weapon" was recovered at the scene and four inmates sustained slashes or stab wounds during the riot.
Two inmates were reportedly taken to the hospital with non-life threatening injuries.
No staff were injured.