Whether they fly over a 4' fence is entirely dependant on their whims. If it is a very large area with plenty of interesting grass and bugs inside, they might stay. If it's smaller, goes to dirt, or they just think something elsewhere looks interesting, nearly all breeds (except silkies) will have an easy time getting over.
I would be equally or more worried about the *dog* going over. He may never have gone over the fence before but flappy squawky foody-smelling "toys" on the other side are a powerful enticement. An awful lot of people have chickens killed by their own dogs, even when they were SURE (and sometimes said so for years, on BYC) their dogs would NEVER touch a chicken.
So, probably some work needs to be done from *both* perspectives.
Good luck, have fun,
Pat