I researched this. What MT is using right now is the Windows compatibility mode, which, on the >100% basically renders to a behind-the-scenes bitmap, then stretches the bitmap. If you look really closely, that blurs things a little. Usually, since on a 4K display pixels are so close together, you really don't notice it. (I had to run high-DPI on a regular display in order to see the effect).
It is possible to change things in MT to run in "DPI-aware" mode, I experimented with it, which would automatically change all font sizes and eliminate stretching. It will take redoing things on almost every window in MT, creating new graphics etc. Right now we have a bit on our plate. Maybe at some point, during a "quiet period"...