Is the iPad Air powerful enough ?
I assume this is the new Air, not the original. If so, yes it can playback 4k. You might have problems streaming full uhd blu-rays, though over wifi. Might have to download or buffer ahead.
4K videos are usually encoded with the HEVC (h.265) codec.
All Apple devices which feature the A9 (and later) chip will be able to utilize hardware decoding when playing these videos. Older devices will need to rely on software decoding, which is less efficient and as a result will limit the playback of HEVC video to 1080p, or lower.
HEVC hardware decode needs A9 processor and above.
10bit AVC hardware decode needs A10 processor and above
Actually, 10-bit AVC is supported on the A9 as well (requires Infuse 5.4 or later). 
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