The developers behind WALTR made this discovery despite nary a peep from Cupertino. Unfortunately though, the 4.7-inch iPhone 6 screen has a resolution of 750 x 1334, and the resolution for the 5.5-inch iPhone 6 Plus glass is 1080 x 1920. This means that the 4K video playbook capability is lost on users of these two new iPhone models. In addition, the size of the these 4K video files would so large that you would be left with greatly diminished storage capacity.
Apple might plan on using the A8 chip for Apple TV, which currently uses the aging A5. If so, there is no question that an A8 powered version would support 4K video.
You can click on the video below to see the evidence that the A8 can support 4K video playback.
source: Softorino via TUAW, SlashGear