A second option would be to use DxWnd. It does not modify any registry files, and runs without installation. While it is generally used to run fullscreen games windowed, it can also be used to change the aspect ratio/resolution of games.
Simply run dxwnd.exe, drag the executable for Avernum 4 into the program, and in the "main" tab, change the window position/setting to your desired resolution and check "Desktop center." In your case, you have a 4K display and Avernum 4 runs natively in 4:3, so the correct resolution for you would be 2880x2160. For a 1080p display, this would be 1440x1080. Keep in mind that this is going to be blurry no matter what, because Avernum 4 runs natively in 1024x768 and increasing the resolution does not increase the field of view and will only stretch, but this will keep it in the correct aspect ratio.
In the "mouse" tab, turn off cursor visibility (since A4 has its own cursor) and turn "Mouse Clipper" on, which locks the invisible windows cursor inside the frame.
Then, keeping DxWnd open, you can either start the game from Steam or Windows. Here is what the game looks like on a 1080p screen with the native resolution.