Sorry you replied while I was editing my previous reply, I was wrong, and was correcting it, the Standard Kodi install uses 320M for GPU.
Ok I have now connected it up to my main 4K TV, before I was playing a 4K source, through a standard hi res monitor.
my TV is
[email protected] as standard. I have tried playing videos which are the following resolutions, through the 4K TV.
3840x2160, 3840x1608, 1920x1080, 1920x872, 1280x720, 720x288, 640x372.
Only one caused any problems, it was an 3849x2160 at an extremely high data rate of 48mbs, (a 571MB file for 1 and a half minutes of video!) I would normally be using about 8 to 10 mbs, it would stutter about every 10 seconds, after I reduced it to a more normal rate it was fine.
As I understand it, the HDMI cable is effectively a high band width serial connection and as such playing full 4K 'may' require a higher quality cable, I know there is a lot of bull shit around cables, so just make sure yours is of a reasonable quality, or try another one.
Otherwise I have the following settings. You may need to be in Expert mode.
I have enabled the White list, in here you select all the resolutions/refresh rates that you want your TV to accept (assuming it can)
so under system->Display->White List, I have selected all the refresh rates that relate to 3840x2160 (my tv raw resolution) also I have set
Player->Videos->Adjust display refresh rate to 'on start/stop' This will phisically change your TV refresh rate, if it is able to, to be the same as the video you are playing, which should give smoother replay, however most of this is fine tuning.
Also bear in mind this is early days yet, I remember trying to play an h265 video on Kodi on a Pi 3, it was managing about 3fps, by the time they had finished optimising, it could comfortably play a 1920x1080 h265, with plenty of spare.

Good Luck.