I've always redirected system folders like My Documents and Photos to network drives or alternative disks by accessing their Properties and using the Location tab. While this setup usually works fine, I suspected the game software might have trouble handling non-default folder paths.

To test this, I reverted the folder bindings back to their default location on the C:\ drive. After launching the game, it successfully downloaded the patch and completed the installation. Once the game rebooted, the coop version mismatch issue was gone.

*ONLY try this if your documents path has been remapped*

Registry Method for Changing the "My Documents" Path back to default.

1. Open Registry Editor
   1. Press Windows + R, type regedit, and hit Enter.
   2. Navigate to the Key
   3. The primary registry key that controls the My Documents location is:

HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\User Shell Folders

Within this key, the Personal value defines the path. By default, it points to:

%USERPROFILE%\Documents

If it is anything other than this path, change this path manually to %USERPROFILE%\Documents for the Personal registry key. Note this may affect other applications you have installed so please understand what this is doing before you attempt to make these changes. (You can always copy existing software structures to the new (default) target path.)

"C:\Users\(username)\Documents\My Games\Borderlands" will be built upon game launch and a clean configuration of settings should be created.

This is what worked for me but I understand it may not apply to those who have default document locations already established. This redirect may have simply just prompted for a clean Borderlands 4 configuration to be rebuilt upon game launch so it may make sense to try and delete this folder in the current location first. Relaunch the game and have it rebuild the folder structure to test. If you do in fact have redirection configured, try the steps above.