As far as I know and read throughout the forum so far, you can't have both the 3.8.1 and the 4.0 versions of the Edain Mod installed in the same ROTWK folder.
But, since I am really not an expert in technical support, I'm sure that you will find other people who could answer you more exhaustively
This thread is a request for technical support, not a proposal nor a topic meant to start a discussion; therefore, I will move it to the
Edain Support section.