iPhone 6S and iPhone 6S Plus are the very first iPhones which are able to take 4K videos. To record 4K video on iPhone 6S and iPhone 6S Plus at 30fps, do as follows:
1. Open the Settings app from the home screen
2. Scroll down to the Photos & Camera settings and open it
3. Find the Camera settings and tap the Record Video option
4. Now make sure to choose the 4K at 30fps option to record 4K videos on iPhone 6S and iPhone 6S plus.