After watching this video https://www.youtube.com/watch?v=bCdOfhQTsWY
I just wanted to ask what your experiences were with 4k playback on the Mi Box. If it can't playback 4k video it's not really a 4k Android TV right?
On the other hand this video says it can playback 4k HEVC (non 10bit) videos just fine. https://www.youtube.com/watch?v=j6V1bh_8m7U
As far as your videos are concerned, I think you are overlooking the bitrate of the videos being tested.
On the first video, at 4 minutes 28 seconds. ( https://www.youtube.com/watch?v=bCdOfhQTsWY&t=4m28s ) He tries playing a video labeled as HD (I assume 1080p) and encoded with h265 (HEVC) at 110Mbps bitrate. It starts studdering a few seconds in and IMO is unwatchable.
Then at 5 minutes 29 seconds, he selects one labeled as 4k h264 with a bitrate of 250Mbps (more than double the bitrate of the first). This seems to play way better, but with a few hiccups here and there...
Then the big one. At 6 minutes and 23 seconds, he plays a video labeled as 4k h265 (HEVC) with a bitrate of 400 Mbps. Before commenting on how it plays, we've already seen what a 1080p h265 video at about a fourth the bitrate plays like... This should be at least as bad if not way worse. And it starts stuttering at about the same time, but seems to chug along a lot more. Though, based on the h264 performance at 4k, I'm more inclined to lean toward this being a performance issue with h265 decoding and the incredibly large bitrate.
Now for the second video with smooth 4k... https://www.youtube.com/watch?v=j6V1bh_8m7U&t=13m35s So, the only video he plays that's explicitly labeled as 4k is also h265, but compared to the first video it's has an amazingly low bitrate at 25 Mbps. It pauses for a second at the start, but then appears to play smoothly.
My reaction to both videos. The first is obviously meant to test h264 vs h265 performance especially at massive bitrates and to compare the MiBox and the Nvidia Shield... I know I didn't highlight the actual side by side comparison... but I didn't because it doesn't seem that valuable. We don't know the specs on the first video shown and I'm skeptical of it as somehow the mi box just "skipped" the bird portion of the clip on it's own, that's generally not what happens when a box is struggling to play a video. It should still play the video just slow and choppy, but not loose relatively big portions of it.
and the second comparison uses the 400 Mbps bitrate and honestly that throws a lot of concern into the mix. It could be having issues simply because it can't read at 400 Mbps from whatever the source file location is... could be issues rendering h265... Too many factors to say it's just because of 4k, I bet it would play the same if it was 1080p at 400Mbps and h265.
So why is bitrate important? well, Youtube recommends 53-68 Mbps for 4K at 60 fps (and most TV/Movies is 24-30 fps). Meaning, while that 400 Mbps is a good test, no one is watching a 4k movie at that bitrate, they are looking at under 1/4 of that. So take the performance with a big grain of salt.
Also as for your question of if it can't play 4k than it's not a 4k android tv... That's correct, but it definitely can play 4k. It can output a 4k video signal, that's really all that's required. Being able to handle a 4k h265 video at 400 Mbps... No one expects that. Being able to handle a 4k h264 at 50-60 Mbps, that's a more reasonable expectation.