Since i upgraded to Windows 10 whenever i started SRIV legacy it started windowed. No matter how hard i tried to change the setting it didn't help. I also noticed that the resolution is greyed out and can't be changed anymore. I searched for a solution and read something about set it to borderless. What you need to do is to go to the installfolder, which is in steam/steampps/common/saintsrowIV (where steam is installed). Then open with notepad display.ini. Change borderless to true and change the resolution to that of your monitor. For example; if your monitor is set to 1900x1080, then use that. Save the file and then launch the game. If done right your game will now look as if it's fullscreen. At least that's what happened to me. No idea if it also works for Windows 11, but one can try that as well.