It's not possible... yet.
1. The iPhone 6S only got 4K this year. The guts of this AppleTV is similar to the 6.
2. It has one USB-C port. I'm going to go out on a limb here and say the port is likely to plug in an external hard drive a-la game console/PVR. Otherwise there isn't enough storage to store a 4K video, and streaming one isn't possible yet because "broadband" is defined as 25Mbps as of Jan 2015, and the average ISP isn't offering 25Mbps as baseline. 4K requires 25Mbps (Netflix requires 25Mbps for UHD)
3. HDMI 1.4 can't do 4K @ 60fps, and Everyone who bought a UHD television before HDMI 2.0a has this, which only came out this year (HDMI 2.0 was 2013 but TV's have a slow production pipe.) 4K@60fps requires HDMI 2.0(a) or Displayport.
Source:
http://www.avsforum.com/forum/168-hdmi-q-one-connector-world/1523994-hdmi-2-0-cedia-webinar.html
Even devices like the GoPro can't do 4Kp60
So a "30fps" experience on a TV is miserable when an iPhone or iPad app/game runs at the screen's native resolution and refresh rate (60fps.) It's a poor experience to switch between 4K UHD and 1080p, and we know the ARM parts aren't really capable of rendering software at 4K. Sure you "might" playback 4K video with the existing ARM parts using the dedicated decoder/encoder, but that does nothing for