I downloaded a few 4K video samples from YouTube and I am now re-encoding them to 2048x1536. Some of the 4K video is actually only 1714p (2.39:1 ratio). So I lost less than 11% of the vertical resolution, kept the aspect ratio and cropped around 916 pixels off the sides. The resulting video is 4:3 ratio, but shows almost the full vertical resolution of the original. I'll see if the iPad will play the "normal" profile (as opposed to high profile) 1536p video...
Edit: iTunes will not sync the 1536p video because "it cannot be played on this iPad". I am now re-encoding the video to 2560x1080p, 1920x1080p, and 1920x800p (original aspect ratio). I'll see if 1080p is the limitation or if 1920 horizontal pixels is the limitation. Hopefully I'll be able to sync the 2560x1080p video.
I'm updating finally, (got around to some free time), I had that 4K video re-encoded to 2048x1536 and 1920x1536 as well as a few other resolutions.
I used Dropbox to transfer the videos (then used AwesomeFiles to play back because DropBox kept the top bar with the "iPad / Time / Battery icon" during playback). I found that the Full HD 1920x1080 worked fine obviously, but the 2048x1536 didn't, not surprised by that, after reading these earlier posts.
However, the 1920x1536 did play perfectly! Playback with both Dropbox and AweSome files, and not even a hint of stutter or lag. The 2560x1080p encoding didn't work either, which was unexpected since 1920x1536p did. I figured the fewer pixels total, the more likely the playback, but 2560x1080 (2.73 MP, does
not play) has fewer pixels than 1920x1536 (2.95 MP, does play). I assumed that the vertical resolution of 1080p would be more limiting than the horizontal resolution of 1920.
Apparently the horizontal resolution is more important than the vertical resolution when exceeding 1920x1080. So shaving off an extra 64 pixels off each side enables 1536p playback of an original 4096x1714p movie trailer re-encoded to 2048x1536.
Does anyone know if this is a overlooked software limitation OR a hardware limitation. (I haven't had any crashing, or bouncing back to the springboard, just DropBox shows nothing, and AwesomeFiles returns to the file list. But both Apps play the 1920x1536p movie trailer fine).