4S Ranch seems to think it is unincorporated “4S Ranch is an unincorporated area in the County of San Diego.”

http://www.4sconnect.com/faq.asp?id=461540&page=3#Q2697

And the San Diego county sheriff seems to think it is an unincorporated area too.

“A substation of the Poway Station, the 4S Ranch Substation was opened in November 2007. The substation serves the 4S Ranch community, an unincorporated area surrounded by Rancho Bernardo, Rancho Santa Fe, Black Mountain Ranch, Rancho Peñasquitos and Lake Hodges.”

http://www.sdsheriff.net/patrolstations/4sranch.html