Most monitors have terrible scaling and so do GPU drivers. Lower resolutions will always look much worse on higher resolution monitors because of it. That's why you always go native resolution when you can. 4K also allows for better post processing effects (using something like ReShade) and better scaling for 2D UI elements (which unlike 3D textures are very obvious when badly scaled). I didn't get into any of that earlier because if I can't get someone to understand the basics, going more into depth didn't seem like it would be very productive.
As for why you didn't get 4K working, did you:
1: Copy DDraw.dll and D3DImm.dll from dgVoodoo2's zip archive to Blood 2's directory?
2: Run dgVoodooCpl.exe , go to the DirectX tab and change the resolution there to 4K?
3: Made sure you're not running Blood 2 or dgVoodooCpl.exe in compatibility mode?
Two things you might ask is, what about the resolution you set in blood2.exe? It doesn't matter as dgVoodoo2 will override it, the only thing it matters for is aspect ratio (so you need the widescreen patch) and UI size. UI will be upscaled without interpolation so you want a resolution set here that would multiple to 4K. I recommend 1280x720 (1/9 of 4K). Higher resolution set here = smaller UI. 960x540 and 1920x1080 are your next steps up or down. This is why you want dgVoodoo2, running Blood 2 natively at 4K might be doable but the UI will too small to be readable.
Other thing is, what's this about compatibility mode? Windows virtualizes the registry, so if you run dgVoodooCpl.exe without compatibility mode and blood2.exe in compatibility mode for Win95, then everything loaded by blood2.exe (including dgVoodoo2's dlls) will search for their settings in Win95 virtual registry. It won't find dgVoodoo2's settings since dgVoodoo2's settings were saved to the real registry since dgVoodooCpl.exe was not running in compatibility mode. I would recommend not using compatibility mode for either exe as its not required. If you have a original disc and not the GoG build see here for how to get music working without compatibility mode:
https://www.gog.com/forum/shogo_mobile_armor_division/fixing_shogos_music_without_compat_mode Some more things you might want to do for the optimal experience:
1: In dgVoodooCpl.exe, in the DirectX tab make sure to uncheck "Disable Alt-Enter to toggle screen state" (might make mouse cursor appear ingame), uncheck "Bilinear blit stretch" (makes crosshair disappear) and uncheck "dgVoodoo Watermark" (pretty obvious). Also set MSAA to 8x and filtering to Force anisotropic 16x if your GPU can handle it.
2: Make sure the game runs at 60 FPS either through framerate limiter or vsync in your GPU driver settings. Higher refresh rates breaks cutscenes.
3: Disable Cleartype in Windows. It will blur and make item pickup texts hard to read.
A lot of work but most of it isn't for 4K, just for getting the game working which really GoG should do out of the box.