Unfortunately, it's not limited to UI scaling - a lot of stuff in the game won't work quite as it's supposed to at that high a resolution (there's a reason games get HD re-releases, etc etc - it's just not that simple). I'd recommend running the game in fullscreen mode at half your native monitor resolution; anything I might reasonably do would be functionally equivalent to that.
The thing you *can't* do now is run in borderless window mode (as opposed to fullscreen) at half the resolution, since it'll just result in a half-screen-sized window. That's unfortunate, and might be something I'll have to deal with eventually. Which is also painful, but less so than making a proper 4k version of the game, which involves things like higher-resolution assets for everything.
(Edit: you might be able to run in windowed/borderless at fullscreen in 4k by scaling your desktop to 200%, though of course doing that for a single game is not very practical.)
Ultimately, though, 4k is simply not supported, and is extremely unlikely to be. 4k *monitors* are supported (through aforementioned "run in fullscreen at half the resolution"), but running at an actual 4k resolution is not.