Hi there! If you've got a 4k monitor, I'd recommend running in fullscreen at half the native resolution of your monitor. I'd like to eventually look at supporting this within the game, but it'd amount to... well, basically the same thing, with worse performance (because it'd be outputting extra pixels, compared to just using a lower resolution), but would work in windowed mode (which would be the point of doing that in the first place).
(If you're wondering, only scaling the UI and not the rest of the game is not a good option, since the rest of the game is also designed to work well within a certain range of resolutions, from graphics assets to zoom levels/scales etc, and that does not include 4k. In-between resolutions such as 2560x1600 are kind of in an awkward spot; some people find those playable in native; personally I'd still run fullscreen at a reduced resolution in that case.)