This is the San Francisco region of San Andreas.

Evidence to suggest that San Fierro will appear in GTA V:

In GTA V's first trailer, a golf course is prominently featured. In GTA San Andreas, there are only two golf courses in the game map. The Yellow Bell Golf Course is in Las Venturas, and the other, Avispa Country Club, can be found in San Fierro. There are no golf courses in Los Santos, in GTA San Andreas.