It's a good idea to train your birds to come to a specific call for a reward of some nice treat (scratch, birdseed, etc.) before you ever let them free range. That way, if they fly or wander off, you can easily get them to come back when you want them.
It's easy to train them: Once or twice a day, go out to the coop and make the call (I yell "chick - chick - chicken" in a high pitched voice), then give them the treat. It's only 1-2 weeks before they learn to come running as soon as you call.
Most mature chickens cannot fly over a 4 ft fence. Slender bodied birds are the best flyers, such as hamburgs, for example. Rounder bodied, short-winged birds like cochins, brahmas, etc. (even if they're bantams) can only hop-fly. I have one hamburg who can easily fly over the fence and even up onto the roof of the house, but she hardly ever does so, because none of her flock mates can follow her! She always returns to the flock after an excursion.
