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I could have sworn i read somewhere that 4K movies can only be played online/streaming? Only 1080p movies can be downloaded for offline play.
Apple does not provide download option for 4K movies.
That does not mean the device itself can/will not play 4K material of your own.
Most modern iPhones can record and store 4K video, and that is easily playable on aTV 4K in full resolution.
 
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