Apple announced its new 4K compatible Apple TV during its press event last week. During its event, it also made mention that it would be upgrading all HD content purchased by customers to 4K. Now that the new media box has gotten into the hands of the masses, it appears that the firm is imposing some limitations on its 4K content that is either purchased or rented from the iTunes Store. The chaps at MacRumors have discovered through a support document that Apple will allow 4K content to be streamed, along with Dolby Vision and HDR versions, but won't allow 4K to be downloaded, as of this time. The firm is very clear stating that:

You can download a local copy of an HD movie, and you might be able to download HDR and Dolby Vision versions, but you can't download a 4K version.

It's unknown at the moment why this limitation exists, but it could have to do with the smaller storage capacities of the Apple TV 4K that currently comes in a 32GB and 64GB variant.