The board only has 2 slots.

The SMBios table does indicate BIOS is seeing both 4GB devices but for some reason is only mapping 4864MB. From looking at my machines I believe Memory Array Mapped...

I'm not an AMD user so I don't know if its true that you're not allowed to boot with only 1 DIMM in the 2nd channel but I I know that's not true with Intel. So I would check to make sure the slot for...

I agree that you should try loading the BIOS defaults.

Can you post a screenshot of CPU-Z showing the 2nd slot SPD information?

Have you tried running MemTest86? How much memory does it show?