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.
None.
I have no idea what to put here... So I guess I'll just put this here.
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.
since a problem like this should be really obvious in test and isn't likely to suddenly develop
Electronics can easily degrade over time from heat damage. Components can easily go from working to not-working.
None.