Hi Barry,
I appreciate for the troubleshooting steps performed.
I suggest that you install one memory module in one memory slot at a time and check to make sure all the memory modules and all the slots are working fine.
Once you have confirmed that all the 4 slots and memory modules are working fine, I request to reset BIOS to defaults. Restart the system, at Dell logo tap the F2 key every 2 seconds. In the BIOS screen, Press ‘F9’ to reset ‘F10’ to save and exit from the BIOS screen.
Reseat the memory and check if it is detected.