I just purchased a unit to fill a similar niche (Minix X8-H) which has recently arrived and been unboxed. It also claims to do 4K video. Haven't yet had time to have much of a play with it, other than verifying that it works. My quick play found it zippy and responsive, although I don't have a compatible screen or any source material to test 4K capability.
This seems like a much better unit than mine spec wise - although mine only cost $NZ 175