The gunman carrying an “AR-type weapon” was reportedly shot by police after ramming his vehicle into the facility in the town of in Red Bluff, 120 miles north of state capital Sacramento, reports The Sacramento Bee newspaper.
After the vehicle caught fire, the man began shooting at workers, according to multiple witness reports.
The four injured persons were currently receiving treatment at the St. Elizabeth Community Hospital.
They are in fair condition, spokeswoman Allison Hendrickson told The Sacramento Bee.
There was also a fire at the facility, which is located in the 10800 block of Highway 99 West.
The ordeal was over in less than 45 minutes after officers shot the suspect, according to radio communications.
It was unknown if the alleged gunman had survived his wounds.
Employees at the centre told local media that a shooting had taken place with about 200 workers inside the facility, some of whom locked themselves in a room.