How to Record 4K Video on iPhone 6S and iPhone 6S Plus
You’ll need a new iPhone with 4K video support to have this feature available, which is the 6S or better:
1. Open the Settings app and go to “Photos & Camera”.
2. Scroll down to “Camera” and tap on “Record Video”.
3. Select “4K at 30 fps” to enable 4K video recording with the iPhone camera.
4. Exit out of Settings.
Now that 4K video recording is enabled, you’re ready to go.