Not many users pay attention on that the newest iPhone generation, iPhone 6S and iPhone 6S Plus, can record video in 4K resolution through the camera. However, this awesome feature actually is disabled by default.
HOW TO TURN ON 4K VIDEO RECORDING ON IPHONE 6S?
Go to Setting > Photos & Camera > Scroll down to Camera > Record Video > Enable 4K at 30 fps.
HOW TO RECORD 4K VIDEO ON IPHONE 6S?
Just record a video at normal except touching “4K” badge” in the corner.