Kodi already supports 4k GUI’s natively, it has nothing to do with being at the skin level.
They say a picture is worth a thousand words so here goes. This is a side by side comparison of a HTPC and OSMC both set to 4k and both using imageres and fanartres of 9999 so no artwork is downscaled when added to the cache.
It’s pretty clear to see that the HTPC looks much better, look and the resolution of the text, the power button but more importantly the poster. The HTPC renders and outputs the GUI at 4k where as OSMC renders it at 1080p and then just outputs it at 4k.
