Divinity Original Sin Enhanced Edition - 4 Player Mod

4 Character Creation For the EE

I don't think either affect GOG/Steam compatibility for co-op.

The pak file should be placed in the My Documents D:OS EE 'Mods' folder; the text file you don't need to keep (but leaving it in the mods folder shouldn't hurt).

The 'Four Character Creation' mod should be shown the list of mods. Is the dropdown selector near the top of the Mods window on "-Main-" (the default)?
Could you have accidentally extracted the zip file into a folder in the Mods folder, or something?

If you have a lot of mods, newly added ones should be listed at the bottom, or if you delete the modsettings.lsx from your profile folder the game will recreate it the next time it is started, putting the mods in alphabetical order (though this will also reset which mods are enabled).

After downloading the 'Four Character Creation EE-84-1-0-0-0.zip' file, extract the 'four_character_creation_c73369ad-3f79-4d0b-9beb-7c221d83c3b7.pak' file to the '..\Documents\Larian Studios\Divinity Original Sin Enhanced Edition\Mods' folder.

For the extracted main.pak file, the only file you need to worry about is the one you edit. It should be placed in the install folder:
..\Divinity Original Sin Enhanced Edition\Data\Mods\Main\meta.lsx