Yeah - I can't keep my chickens out of the preschool playground next door. They hop over my fence (4' on my side, 6' on theirs) and get back over without problems. One of my hens frequently (and irritatingly) flies up to nest on top of the 8' roof of our coop. All with clipped wings. :|Mine CAN fly but rarely do. Tuck the wings in tight and run is the primary mode of transportation like they don't realize they can fly faster. My understanding is that chickens USUALLY don't want to fly over something they can't see the other side of so they go to the top of the fence first. One technique to stop them landing there is to string a wire a few inches above the top since they can't land on it. Of course if they WANT to be on the other side, that won't stop them, 4' is not an issue for a chicken.