I've created a (unpublished) Mod which upscales the UI (i.e to use a 4k resolution with an UI like 1280x800).
Unfortunately at least two issues made it still unuseable:
1. Not all Elements will be scaled, but the Python based only. The Diplo-Screen, some popups, etc will be very small.
2. Font and Icons (GameFont.tga) can be scaled but the use different baselines. => The Icon position will be wrong on big font sizes. (Solution could be an special font?!)