Just to be sure, have you checked in the bios that you don't have 4 GB of RAM assigned to integrated graphics (if enabled)? It doesn't really sound like it based on the other things you have said, but I just wanted to mention it since no one else did.

When I get the money (Which may be a long time from now since I'm running everything up to my standards right now) I can give an update on if it actually is either the CPU or motherboard... Or if I can find friends with parts.
They got rid of the memory remapping feature in current-gen motherboards...
Also, it's both slots 3 and 4 messing up. Even if I leave it empty, it still says the last stick of RAM is unusable.
Right now I'm sitting with 16GBs of usable RAM, so I'm fine until I pay off my debts and test the hardware out.

Electronics can easily degrade over time from heat damage. Components can easily go from working to not-working.