The next update planned for AppleTV set-top-box later this year won't likely support 4K videos atleast not in the initial roll out. It may come as a shock but it makes sense as 4k technology is still in its fancy. The Apple TV update is estimated to do many things such as run apps and stream a TV service but the device will stick to 1080p.
The A8 chip to be shipped in device can handle ultra HD 4K resolution but according to Apple market is just not there yet. Moreover limited 4K content is available for view such as House of Cards and a few movies in 4K on Netflix but you need to have an 4K player to stream the same.
Also in addition to enjoy 4K streaming consumer need to have an high speed internet connection and a 4K television set so once 4K content and streaming becomes viable we hope Apple will be there.