Well, my Win10 computer that had Steam on it, with SH3 with a WAC and GWX mods, and SH5 with a TWoS mod, have been obliterated by a Windows "Update". In fact, the computer will not boot now, so I cannot go in and look at what I did, so this is from memory...
What I did was added a Library folder to my Steam, but since it is a laptop with just one hard drive, I had to do the "manual" method, as documented in
Moving a Steam Installation and Games on the Steam web site. The relevant part would be the "How do I move my existing Steam Installation?" if you only have the one drive also. It does look like they've updated that page recently, and one of the little outlined boxes now makes mention of the Steam Cloud. That was not around when I did my move. Also, this process takes ~forever~ and a day. The bigger the Library, the longer the move.This will also mess up any short-cuts you have on your desktop, so those will have to be either edited, or deleted with new ones created from the Steam Library. If you have a 2nd hard drive, it is easy-peasy, just by creating a New Library folder. You can then pick and choose what you want to move to that. That is the top part of that article. You do have to get the library out of the Program Files folders though, for mods to function correctly.
Once you have a Library outside of the Program Files folders, You can then copy your SH4 install. What I did with my SH3 was to copy the "Silent Hunter 3" folder and pasted it to the "Games" folder where I had made my new Library. That takes a bit to copy. While there, I renamed that folder to SH3GWX, to represent the mod to be applied. Then I cut it from the Games folder, and pasted it back into the "common" folder under "Steam / steamapps / common /", such that I have "Silent Hunter 3" sitting alongside "SH3GWX" folders in the "common" folder. I then used MultiSH3 to make a new Save folder for the GWX install (GWX). Then I brought in JSGME and the mod, activated the parts of it and got it ready to run. Then I went into the Steam app on my computer, clicked the "Games" menu, and then "Add a Non-Steam game". "Browse" to the folder where that SH3.exe file is in the SH3GWX folder, and click "Add to Library". After that, change the name of the shorcut to match what you want, which in my case was SH3GWX. To do that, right-click on the shortcut in the Library, left-click "Properties", and change the text in the top box (sh3 in my case) to what you want (SH3GWX here). When you press the <Enter> key, it commits it. So now I have an SH3GWX and an Silent Hunter 3 shortcut in the Steam Library, each pointing at a different SH3.exe file and folder. Just substitute "SH4" and mods as applicable for you. The MultiSH you'll need to use would be
MultiSH4...
