Since there is a lot of talk about how much better WiFi is than 4G or even 3G, I thought I'd run a little test just to show it....
On day 1 I ran my phone on mobile data all day, no WiFi, a mix mostly 4G and some 3G. On day 2 I ran my phone in typical fashion. That is WiFi almost all day - 13hrs out of 15hrs - with 2 hours on 4G. Both days had strikingly similar usage, almost exactly 1hr of screen time, no streaming. The only usage difference was that on day 2 I had about 40min of voice calls, whereas on day 1 it was only 20min. Obviously a pretty mild day usage-wise. Take a look at these charts and then imagine what the 4G would have been like had I spent a couple of hours streaming!
15 hours on 4G = 3% remaining
13 hours on WiFi, 2 hours on 4G = 43% remaining
The moral of the story is, obviously, use WiFi whenever you possibly can!