I looked at the electric fence, but what keeps the chickens from flying OVER it? I have a 10*10 covered pen area off the coup that they have been living in. Before we put a roof on it, they flew out. It is 10 foot tall. Half the flock just flew out. So why wouldn't they fly out of a 4 feet high electric fence? Or is there a different one out there that you are talking about?
They're not smart enough.
Chickens don't seem to be able to figure out wire fencing - they'll run back and forth along it confused for hours - put a top bar on it and they go right over it.