Luis Patino, spokesman for the federal receiver who runs prison medical care, says tests have come back on four of the 30 inmates showing flu-like symptoms.
The tests show a 97 percent probability that the inmates were sickened by the H1N1 virus, but confirmation will take 10 days.
Tests are under way on the other ill inmates.
California ended visitations at all state prisons for 10 days in May after an inmate at Centinela State Prison in Imperial County became ill.
Patino says this time officials are isolating inmates only in one cellblock at San Quentin, north of San Francisco, because they know better now how the flu affects humans.