How to turn on 4K video on your iPhone 6s
4K video is a fancy name for 2160p, in other words twice the resolution of 1080p HD. That's a lot of pixels and it can take up a lot of storage spaceŚ300-400 MB of storage per minute of videoŚwhich is why Apple leaves it off by default on the iPhone 6s and iPhone 6s Plus. If you do want to shoot in 4K, however, either for the full resolution or just so you can pan, crop, zoom, or stabilize your 1080p, it's easy to turn on.
Full story from the iMore Blog...