The discovery was made by the developers of WALTR, a Mac app that lets users upload unsupported video file types to the iPhone.
While testing the app developers found it is possible to playback 4K videos on the iPhone 6. Even if Apple hasn't announced it, the iPhone 6 and 6 Plus are ready to play 4K videos.
The iPhone 6 and iPhone 6 Plus have resolutions of 1334x750 and 1920x1080 so they obviously can't play the video at 3840x2160; however, it's nice to have the ability to playback 4K if need be.
The company has posted a video to YouTube with proof which you can view below.