a friend of mine just got himself one of these 4K Monitors for his PC.
He installed Rhino and complained about the small icon size. (Seems like the actual area to hit a button is approx. 3 x 3mm)
I told him to use keyboard shortcuts instead.
But maybe these High Resolution Displays are something to keep in mind, when / if you are planing on reworking the user interface.
I realize this is a Serengeti post, but is your friend using V5? I put a lot of work into the Serengeti build to auto-scale the UI under high resolution displays and am a little surprised to hear that this is an issue. If your friend is using Serengeti, then please put me in contact with this person or send me some screen shots.
With a System Registry key addition and copying a manifest file to the folder where 64-bit Rhino V5 lives, you can fix the small icon problem on high resolution monitors.