If installing 4.0 becomes mandatory to play 3.8, this may be what I do - move it to an external drive (Steam makes this pretty easy), assuming I continue to prefer 3.8 over 4.0. Hopefully I'll be able to actually launch 3.8 without 4.0 "plugged in".
Though it dawns on me that I might have to do this manually, since Steam will probably think of 3.8 and 4.0 as one big single Elite install. A basic move of the 4.0 subfolder with a symlink in the main install directory should work.
Yeah that's why I used junctions. That way you can move just one folder of the install directory (the one steam puts 3.8 in) to another drive and to your operating system it's transparent.
Junctions are a native function of Windows, it's a bit more of a techy power user technique, but here's the Microsoft learning article on it:
https://learn.microsoft.com/en-us/windows-server/administration/windows-commands/mklink (Yes I know it looks like you already know this, but I'm also posting for the benefit of others)
Basically process is cut/paste the folder in question to the new location then, then delete the folder in the original location. Finally go in an admin-level command prompt, cd over to the original directory and do a "mklink /j foldername D:\other\drive\path\to\folder" and it'll make a junction that tells the operating system "hey, this folder is actually over here."
There are some tools out there that can do it for you with a more user friendly interface, however I can't speak on any of them as I just use the native command prompt.
I can at least personally attest a /j will work. not too sure on a symlink though, probably, but I just generally use /j instead of /d.