Same as Xbone, OS uses them. Presentation of KZ:SF around PS4 launch showcased that developers then had access to 6 full CPU cores.
As for yields, PS4 GPU has 20 GPU CUs [same as 7870] but 2 are shut down for yields. GPU is very big, and chance that error will land on GPU is much larger than error hitting CPU.
http://www.extremetech.com/wp-content/uploads/2013/11/ps4-reverse-engineered-apu.jpg
Yep, I know that games only have access to six cores on PS4, I was just wondering if, of the two cores not available for games, one was marked for redundancy.
Without snap and the NUI it's pretty clear that MS could make do with only one reserved CPU core. As Sony have neither snap nor Kinect, it seems that they should be able to make do with only one core, leading me to wonder about the possibility of a redundant core.