Side note: only 4K (4000x2250) movie was to slow. Music was correct but video was slow, hanging and jumping. For me that is no issue, as I don't have a 4K TV anyway.
Ordinary 8-bit 4k should work, but the issue is in buffer/memory management (not enough CMA memory), so it falls back to SW decoding (press "o" during playback and check info). I'm working on IOMMU support, which would solve this issue completely, but it seems IOMMU driver doesn't always work correctly. This will take some time to solve.