Can I play the 4K video I shot by iPhone 6s on itself?

Well, if you want to play 4K videos, you have to play it on a screen that it has enough resolution. Of course you can play 4Kvideos shoot by iphone6s on itself, the videos can not be played in real 4K because of resolution limitation(6 1334*750,6s 1920*1080). And the resolution of 4K videos is up to 4096×2160. Therefore, if 4K videos are player on iphone 6 or 6 Plus, the dedicate image is unable to be presented. So my suggestion is to play 4K videos on UHD media player.

If you are in need of it, try this 5KPlayer. I've used it for several months to play 4K and 5Kvideos. It plays 4K videos fluently and shows a very beautiful image.