I live in Sacramento,CA, the house is block ext walls (hollow concrete blocks, not filled).

I'm looking to have a 12" x 12" footer and using rebar and I'd like to go with 4" on the inside after the footer.

Is 4" thick enough?

 

I'm gonna say you need to check if any local building codes apply.
I personally think 4" is thick enough, if properly reinforced. But if you shirk codes and there is an issue for the buyer sometime down the road after you eventually sell, it'll likely be your responsibility.

 

4" minimum down here, poured over a vapor barrier and 2" of sand.

But like ckal said, you had best consult the building department. 12x12 most likely will not be an acceptable footing for a 8' concrete block wall, and the walls will need to be reinforced and grouted, not hollow.

 

4" is the norm, but like said before, check your local codes. Cali codes are a lot stricter than Pa codes due to the earthquakes, mudslides, fires, locusts, etc you guys have out there

 

Down here in florida when we built my shop we were required to have a footer that was 24" DEEP, and that was with stud construction on top of it. Talk about overkill!!! We did go with a 4" monolithic slab with the footers. Good luck.