You don't need to change the roaming folder. All edain parts are in the folder in the program path.^^
You can use this strategy:
Install Rotw, patch it to 2.01.
Install 3.81.
Copy the folder to another place and rename it to 'Edain 3.81'
Afterwards install 4.0. The installer will always look for the original folder (original path, original name), therefore it won't changed the copied folder.
The exe in the copied folder will start 3.81, the other one 4.0