How and when to use 4K instead of 1080p video on iPhone 6s and 6s Plus

The iPhone 6s and iPhone 6s Plus were welcomed by cheers from the audience at Apple's busier-than-usual September event. The iPhone 6s boasted interesting new features including an all improved camera with a new sensor that meant the iPhone 6s and iPhone 6s Plus were able to record in 4K at 30fps - but what is the difference between 1080p and 4K? Here, we outline the differences between the two, how to switch between them on your iPhone and the best conditions for recording in each resolution.