A child's playhouse or something about that size would be ideal for four chickens. If you are going to try to keep them in with a four foot fence, though, you will want to keep a wing clipped on each chicken, and plan on them flying over from time to time. We have a 6 foot cedar fence, and our two leghorns were able to fly up and perch on it with just a little help from a playhouse we kept a bit too close to the fence. If they really want to get up on the fence, they will find a way. Still, our chickens haven't tried since then, so, maybe yours will be good about staying in the yard. If you are doing mostly chicken tractor ranging, and you are out to monitor them when they free range, you may be fine with the fence you have.