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I didn't think the iPad Pro supported 4K resolution, so why would you want to have a movie in 4K taking up all the memory/bandwidth?
 
You need a 60"+ screen to see the difference between HD and 4k. Offering 4K makes more sense for Apple TV users, except the current Apple TV does not support 4k.

For the iPad Pro? Irrelevant.

I can easily tell the difference between 1080p and 4K on a 13" laptop. Why do you think you need a 60" TV to see the difference? For reference, I have a 65" 4K TV, and at 15 feet with decent content, the difference is obvious over my 1080p 65" TV it replaced.
 
You need a 60"+ screen to see the difference between HD and 4k. Offering 4K makes more sense for Apple TV users, except the current Apple TV does not support 4k.

For the iPad Pro? Irrelevant.
My 4K videos from my GoPro played on a 2015 15" MacBook Pro definitely look much cleaner and sharper than the same video run in 1080P on the Retina display.
 
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