A four-foot fence is a piece of cake to those lighter breeds and young'uns with a substantial wing span. I've seen hens launch themselves in one attempt to fly up to barn rafters and made it. I, personally, would have some type of netting over a run to keep them contained. I would consider them as pheasants, taking off in a hot minute, and landing in a neighbor's yard with a dog, to meet their demise. --BB