Thanks for your reply.
If I put DIMMS in slots 1 and 3 then it shows 8GB. If I put DIMMS in slots 2 & 4 then it shows 8GB. If I put DIMS in slots 1 & 2 or 3 & 4 it only shows 4GB.
All slots how active in the BIOS monitor section and CPU-Z finds them but both the BIOS and the OS only show 8GB...
Same issue with my P8Z77-V Pro. I also lost the ability to change the turbo ratio with the 1206 BIOS update so I'm guessing it's related.
I know Asus support told me a future BIOS update would fix the Turbo Ratio issue so I'm thinking it could fix this too.