Hi marzRX, you might try the control panel method for setting associations, the 2nd method mentioned here:
https://fileinfo.com/help/windows_7_file_associations

Another thing you can try is to uninstall v4 and then re-install it, the installer should set the file associations.

There is also a new v5 beta that you can get at https://moi3d.com/beta.htm .

If nothing else works then the place to set it in the registry is under:
  HKEY_CLASSES_ROOT\MoI.3DMFile.1\shell\open\command
which will have a value like this:

"C:\Program Files\MoI 5.0 beta Apr-8-2023\moi.exe" "%1"

You'd change that path to the v4 moi.exe

- Michael

Thank you, Michael. I was able to associate the extension by modifying the registry key as you suggested.

I chose to edit the registry key because I was worried that settings like CustomUI setup would be lost if I reinstalled v4.
(I had already tried both methods 1 and 2 before asking the question.)
--
marzRX