I'm using 42" 4K monitor.
For me 200% sized skins results in way too big GUI elements. it seems 150% might be perfect for me.
You can see on screenshot below, that 200% skin (Comala) leaves not too much space for plugins. Of course, it can be changed at expense of changing sizes other GUI areas. Also there is limited number of visible tracks in mixed as well as in tracks area. This is not what I expect from 4K monitor.
That's why I stick to a skin "Default 5 Dark Extended", with font sizes modified by me to required sizes.
Unfortunately this skin offers no as many layout options as others, lets say Comala5.
At the end everyone will find something which matches his needs/workflow.
BTW: In Reaper preferences I have High Dpi mode set to 'Aware'.
Click for original size pictures
with regards