One of the ram chips probably has failed. Run the memory test.  How to Test Your Computer’s RAM for Problems (howtogeek.com)
I have 2 Optiplex 9020 PCs that were purchased at the same time with identical specifications. Both came with 8 GB of ram. They are both running Windows 10 64-bit. At some point last year, one of the PCs began reporting that it had 8 GB of RAM installed but 4 GB was usable. I compared that with the other PC which reports Installed RAM - 8 GB. I've searched and found discussions about relatively small amounts of RAM that are used by the integrated motherboard video but not half. Anyone know what might be my issue?
Memory Issue with Optiplex 9020