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Can I play a 4K movie on this iMac, and see the difference in 4K?
Yes. The display resolution of the 5K iMac is higher than 4K, but going from 1080p to 4K still makes as much of a difference as it does on a 4K display.

4K content availability is another issue though. iTunes doesn't support 4K movies in MacOS or Windows, and Netflix + Amazon Prime only support it in Windows. The only device Apple makes that supports 4K from these platforms is the Apple TV, and it's pretty sad to see the Mac excluded for so long despite being fully capable of 4K playback.
 
“iTunes doesn't support 4K movies in MacOS”

I was looking in the iTunes Store in macOSX on the iMac, and 4K was available, but are you saying I can’t download an iTunes movie in 4K?
 
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