The iPhone 6 uses an 8MP sensor with a 4:3 aspect ratio. 4K video is 8.29MP and uses a 16:9 aspect ratio. So its not possible on the current iPhone range. The 4:3 aspect ratio 12MP sensor on the upcoming 6S/6S Plus allows for 4K video using the 16:9 aspect.
7/2/15 at 9:11am