First, Apple hasn’t said how many 4K HDR titles it’ll have for purchase or upgrade, but we can probably get an idea by looking at other services. Vudu offers 121 movies in Ultra HD, compared to more than 24,000 films in standard or high definition, while FandangoNow lists 197 Ultra HD films, so it’s safe to assume most movies will remain in HD for a while. Disney has also indicated that it won’t offer 4K HDR movies on iTunes at all.
The major advantage that the new Apple TV 4K will offer which no other streaming box has, is that is now has all the major places and services of where to Watch content. Other streaming boxes like Roku and Amazon may offer Netflix, HBO etc but none of them can offer all of those services and iTunes content. When Amazon comes to Apple TV later this year, then this really is the streaming device that will have all the services included.