I do this occasionally. I just wish Apple made it easier to turn off and on permanently - as is right now you have to go into the settings page and flick a switch.

Though, given my terrible memory this particular solution sometimes comes back to bite me in the bum: Usually when I turn off wifi I forget to turn it back on again until I’m at work and I’ve already downloaded a few hundred megs of music, all the whole stupidly ignoring the 4g connection icon.