You can't create a new Library folder in the computer Steam app, ETR3(SS), copy your other SH4 into the new folder, and then "Add non-Steam game" on the Steam app menu with SH4? That's how I did my SH5, and had a Stock version (ee-yuck) and a TWoS install. I do not have a Steam SH4 to experiment with. Oh, wait... the "base" is SH4, v1.3. I wonder if that has anything to do with it...
I do know that Steam did change their DRM a while back, after I had done my first installs of SH3, and that the last time I went to move my Library on the laptop (one hard drive), I had to move ALL games in the Library to the new location that was not in "Program Files (x86)", in order to get the one (SH3) to move. I did not use the same folder name "Silent Hunter 3", but instead made it "SH3GWX". I then went inside the SH3 original folder, copied the "Data, DirectX, Support, etc. folders and all the files (use <Ctrl><A> to select them all), and pasted that inside the "SH3GWX" folder. After that I was able to make copies of the SH3 install, change the name of it to the mod I was running, and then add it into Steam's Start menu. Use of MultiSH is practically mandatory.
