Welcome to SubSim Nico341256!
I also have a 4k monitor, a 32" behemoth (for me anyway), and run SH3, SH4 & SH5. With SH3, the widescreen applet I'm using is set at 1920x1080. My desktop is set to 125% to make the icons big enough for me to read them. What I have done in my set-up is created a desktop short-cut to start my SH3, but I don't start the game until after I have minimized or closed all Windows. Once the game is loaded and I am either into the Single Mission, or out on patrol in my career, I will then pause the game, and use the Windows key to get to my desktop and go from there. Minimize your desktop windows again before going back into the game. That's probably what you do already anyway, but that is about all you can do. With that "methodology", your windows usually will not re-size when you close SH3. I have to set my SH3.exe "Properties" on the "Change high DPI settings" button's window in the top section to use "Program DPI" and set it to "I open this program" in the drop-down. My SH4 and 5 are set to run at 2560x1440 because I cannot read the dials and text at 4k, and have the same basic issue with the regular Windows desktop windows re-sizing when I close SH4 and 5. The same technic there, in using the desktop shortcut to open the game after minimizing or closing all the regular Windows apps. The Silent Hunter series of games were not written to the 'modern' Windows GUI, and do not have a clue about "sharing" anything with the rest of the computer. In fact, shelling-out to Windows while running the game can cause trouble in the game when it get the focus back from Windows. If you do notice graphic or audio anomalies doing that, Save your progress, exit the game, restart the game and load your Save, you should be back to "normal", or as normal as Windows and SH3 allows...
