[sanity]

his inconvenience , Maybe it's due to incorrect settings for your 4k panel , in madVR (Devices>Display Modes setting up  2160p60, 2160p59, 2160p50, 2160p24, 2160p23  instead of  1080p60, 1080p59, 1080p50, 1080p24, 1080p23  in my opinion it should be this the problem that  Kodi shrinks to a little box in the top corner of your desktop

 Thanks for you input, I have it set as you described  ( i.e. 2160p60, 2160p59, 2160p50, 2160p24, 2160p23 ) also the screen resolution is automatically set to UHD res in Kodi

Thanks guys for all your help 

I also have this issue with my setup. 
when I stop play back of a 4K movie Kodi shrinks to a little box in the top corner of my desktop but it's still controllable.

I have the settings suggested here in madVR but it didnt do anything.
One thing i have noticed is if I press TAB and then stop the movie it reverts back to kodi in the correct fullscreen mode.