As long as there is no rail at the top of the fence for them to hop-fly onto and perch on, 4 ft is adequate even if you do not clip wings. Only the flightiestof breeds, like hamburgs, will venture over it. If you have just few such birds in your flock, they won't fly out anyway, because they want to stay with the flock.
I have one hamburg in my flock of 19, contained within a 4 ft wire fence. She can easily fly onto the roof of the house, yet hardly ever flies out of the chicken yard. When she does, she soon returns, because she wants to be with her flock mates, none of whom are inclined to follow her on an adventure. I know that some of them can clear a 4 ft fence if they want to, but I guess it's such an effort for them, that they just don't bother. That's how it is here, anyway. Even my bantams don't try to clear it, once they reach about 12 weeks of age.
