A thing to remember is that it only takes *one* hawk, or (equally likely) loose dog, and you could be down to just a few piles of feathers.
Also most chickens are perfectly capable of flying over a 4' fence... whether they DECIDE to or not is another matter, but that can't be predicted in advance, only retrospectively as you are chasing your now-loose chickens, or a raccoon is eating them.
If you are good with that possibility then that is totally fine; I'm just saying, don't decide to leave the run open-topped under some illusion that there is no threat involved. Browse some threads in the "Predators and Pests" section of BYC to see what I mean.
Good luck whichever you decide,
Pat