TUAW reports that the iPhone 6 and 6 Plus are both capable of playing 4K video files, although not at full 4K resolution.
The devices are able to run and play the high resolution video smoothly without interruption, the only limitation being the fact that the displays have resolutions of 1334 x 750 and 1920 x 1080 pixels, bellow the 3840 x 2160 resolution of 4K. TUAW says that the discovery was made by the developers of WALTR, an app that uploads and converts pretty much any video of any format for native playback on iOS devices.
Source: TUAW
Kind of a lie. The native/standard playback infrastructure still can’t play 4k content and is locked to 1080P. Even my iPhone 5S can play 4k video with GoodReader app. If you want to break the 4k limit you really need to use a 3rd party library, like FFmpeg to render instead of AVPlayer.