Fourth of July in Sonoma County

For 2012 the Fourth falls mid-week. Many wineries close for the Fourth and most of those that are open will close early that day (maybe 3 pm). The number of visitors is usually low as folks are going to the beach and/or a BBQ. This year with a Wednesday holiday expect most wineries to be closed that day. If you wish to visit a few tasting rooms then call ahead to see who is open.

Otherwise, there are lots of fireworks shows in the county starting on the weekend before the Fourth. A couple of towns have parades on the Fourth. Also, there's the Napa County Fair in Calistoga. 
A list of fireworks shows from the Press Democrat.