You may not realize it, but the new Apple A8 processor on the latest iPhone 6 and iPhone 6 Plus can handle 4K video playback even though the smaller Apple phone has a native resolution of 1334 X 750 pixels while the larger Plus model has a 1080p Full HD display with 1920 X 1080 pixels. Though you won't get a pixel-for-pixel output on the phones' screens, the A8's ability to handle 4K playback means that users who sideload 4K content would be able to still watch it on their phones.
Discovery of the ability to play back 4K content on the phone was reported by TUAW:
The iPhone 6 and 6 Plus can play 4K video right out of the box thanks to its A8 chip. The discovery was made by the developers of WALTR, a great Mac app that allows users to quickly upload video files to their iPhone which aren't supported by iTunes, such as FLAC and MKV files. While testing the app developers found it is possible to playback 4K videos on the iPhone 6.