I had some old PC parts laying around and I got this used motherboard Asus M5A78L-M PLUS/USB3 to build a working computer. But this motherboard seems to have some issues with RAM. I installed 4x4GB sticks of RAM totaling 16GBs. When all 4 sticks are installed, then the PC will freeze while booting. Removing 1 or more sticks from any of the slots will make the PC boot but only 4GBs of RAM shows up in BIOS. Lets say I have 3x4GB (12GB) of RAM installed then BIOS shows only 4 detected and 3,8 usable but Windows reports that 12GB of RAM is installed but only 3,73GB is usable.

I am using 64bit Windows 10 and according to manual this motherboard should allow up to 32GBs of RAM.
CPU is AMD FX-6300
RAM sticks used:
2 x 4GB Kingston KHX1600C10D3
2 x 4GB Kllisre KRE-D3U1600M

I have tried swapping out stick or using just 1 stick in any of the slots.
Checked the max memory config in Windows under “msconfig” > “boot” > “advanced options”
BIOS has the latest version installed.
Monitoring tools like HWINFO64 will also detect all the RAM sticks that are installed.
But for some reason only 3,73GB is usable?!

Has anyone faced similar issues before?
Any tips on how to fix this or what to debug next?