It seems that iTunes Store is ready to market an Apple TV fifth generation. The mention “4K, HDR” has appeared in the history of buying or leasing certain movies, such as Passengers or Fantastic Beasts and Where to Find Them.
Even though the purchase history refers to 4K HDR versions, they are not yet available, as MacRumors notes. Only SD or HD versions (720p or 1080p depending on the movies) are offered, the same ones that are supported by the Apple TV 4 currently on sale.
These 4K (ultra high definition) and HDR files will probably have to wait for a fifth generation Apple TV capable of decoding it. But the rumor has been running for several months about a future box, code-named J105, supporting 4K HDR films. Also, Tim Cook said that a new Apple TV could be presented in September.