OK, I found a fix that allows one to play Starsector in full, 4K glory. If you want the explanation of
why it's goofy on some people's 4K displays, open the spoiler; otherwise just skip to the pictures and do that.
Spoiler
Basically, the problem with 4K that most of you will experience in Windows 10 has to do with Screen Scaling- a feature that was introduced because, well, 4K screens that aren't giant have such tiny pixels that merely seeing things has become a problem (especially for old fogeys like me).
The default Screen Scaling setting in Windows 10 for 4K displays is 150%; that means that all text and graphics are scaled up. If you have Screen Scaling at 100%, probably none of this applies, but I didn't test that.
However, SS, started up in 4K resolutions after a clean install appears to be getting data from Windows 10 indirectly; the query to where the center of the screen is, XY, is off. This makes the launcher, the game's window, and many other elements offset inappropriately to the right and downwards, respectively. The fix below corrects all that.
First, right-click on your Starsector shortcut icon, and select
Properties.
Now, click on the Compatibility tab:
Click on the button labeled "Change high DPI settings".
Now change the settings there to look like this:
Open the game. The Launcher should be dead-center on the screen now. 4K should work, either full-screen or borderless window.