It won't work
...until H.265, which promises to stream 4K with the same bandwidth as we use for 1080p now. Also, the truth is, the iPhone 6 and 6+ is completely capable of playing (but not displaying) 4K at 60 fps, as was verified by the developers of Waltr, the incredible transcoder that uses the iPhone 6 hardware encoder to produce brilliant H.264 at a speed the software I use at work can't touch -- 10 minutes for a complete copy of a feature film, for instance.
There may or may not be 4K in the next Apple TV. It's got to be ready, software and hardware. Apple doesn't want to put in a knockout new feature that people won't notice, or will just find to be slowing everything down. Part of the problem is the bandwidth it needs from the network. H.265, better firmware encoder/decoders, better screens, and a faster net. If something is shot on film, it transfers to 4K beautifully. Shot on 5K Reds, amazing. When we have screens with better contrast, we've got a new medium. Until they want footage for VR. Hey, somebody needs to invent a camera! Greater definition and color depth, great. 3D, yawn.