How hard they try to fly over the fence depends a lot on how happy they are in their run and what's on the other side of the fence! If they have a spacious run with plenty of room so they are not feeling crowded and getting on each other's nerves then they may not attempt a four foot fence. But if there is green grass or anything else outside the fence that catches their fancy they may give it a shot. Also, if it's to small and if there is ever any chasing or bullying they may try to fly out.
My run is made out of 48" chain link dog run panels, I solved the flying over issue as well as the hawk issue by covering it with a heavy duty shade cloth custom made to fit my run from my local hardware store.
My run is made out of 48" chain link dog run panels, I solved the flying over issue as well as the hawk issue by covering it with a heavy duty shade cloth custom made to fit my run from my local hardware store.