WINDOWS 10 ONLY
Although I don't have a 4K monitor, I do run Studio One 3 on a Surface Pro 3 (2160x1440 screen) and suffered this same problem which I have overcome. I can only comment with respect to Kontakt, Reaktor, Maschine 2 and some Waves plug-ins but the steps below solved the problem with these for me.
Surface Pro 3 running Windows 10 Creators Update set to max resolution of 2160x1440 with desktop window scaling set to 200% (this is my normal setup for everyday life).
Screenshot of Studio One running Kontakt BEFORE:
Screenshot of Studio One running Kontakt AFTER:
The fix is to change the Windows program compatibility settings for Studio One as shown below to override high DPI and set to System (Application isn't sufficient). Easiest way to get there for me was to right click on the Studio One icon on the taskbar when it's running, right click again on Studio One from the menu popup and then select Properties. Obviously, you'll then have to restart Studio One
As I said, this also worked for me with Reaktor, Maschine 2 and a Waves plugin.
You may find that you need to resize your plugin in Studio One to its smallest before you make the change otherwise you might not be able to reach the edges/corners to resize afterwards as they are offscreen (this happened to me with Kontakt). In fact, although Maschine works, even at view size set to Small it still overshoots the screen by about 1cm.
As a bonus, the same solution when applied to Notion 6 sorts out the plug-in size problem there as well.
Of course, I agree with everyone else that Studio One should, at a minimum, apply the same scaling it applies to the rest of the app to plug-ins as well but a custom definable one for each one would be even better (this plug-in problem also does not happen in Cubase 9.5).
Anyway, hopefully this might help a few other people.