READ the START HERE thread where such 4K capable Kodi devices are already listed - and yes they can output 4K video at 4K.
I would not use the Mi Box for high bitrate 4K Kodi video streaming - it being WiFi only, you really need Ethernet for 4K streaming reliability.
99% of 4K Android boxes have to be manually switched to a 4K Android Settings output resolution if you want true 4K video output.
4K Linux / LibreELEC / OSMC (and I believe 4K Windows) boxes can Autoswitch from a resolution of 1080p <<---->> 4K on the fly depending on the source video content being played.
Why would we want to run a 1080p system resolution most of the time and Auto switch to 4K only for 4K video content ?
Because modern 4K TV's upscale 1080p > 4K video content and produce higher quality picture results vs. letting relatively cheap 4K media players doing the upscaling.
If you are a demanding Kodi user and want optimal quality 1080p & 4K picture outputs you have to manually switch Android Settings ALL the time.
Have a good read
HERE where demanding 4K NVIDIA Shield owners complain about poor quality 1080p > 4k video upscaling on their Shields vs. what their TV's can do.
4K Android box sellers hope you don't notice such shortcomings "too much" - In the case of Android TV devices you are buying such boxes for all round Apps usability and convenience, which is exactly what a lot of people really want.
There are better 1080p / 4K options available if you are a demanding Kodi user.