The fighting involved as many as 100 people at Shoreline Amphitheater.
At first police thought three people were taken to the hospital but learned Sunday that one more flagged down an ambulance that was on its way to the hospital, and two others walked themselves into the hospital for treatment.
Police responded to the Shoreline Amphitheater around 9:30 p.m. after reports of stabbings. Mountain View police asked for mutual aid and officers from about seven different agencies showed up from as far away as San Francisco.
The officers went into the venue, many wearing riot gear, and broke things up. At that point Shoreline officials went ahead and brought down the lights and ended the show early.
"I hear it was very unruly. Unknown if there was alcohol involved, but there was a gang presence. A lot of times when you have people from those different gangs, they're going to have problems," said CHP Sgt. Mike Martinez.
The show was put on by KISS radio station and WAR was the headline band. The group is known for its hit "Lowrider."
Police were called to the scene the first time at 6:30 p.m. to break up some fights. They did that and left. A bartender working Saturday night told ABC7 that at that point Shoreline officials told them to slow down the number of drinks they were serving people. But, things turned violent again at 9:30 p.m.
No arrests have been made. Officials say the victims are not suffering from life-threatening injuries.