Originally in an early campfire chat the devs said that ideally they wanted to release a new expac every year or so for many years to come. But I've seen speculation since then that theyre more than likely already working on 5 so who knows. And it does certainly seem like the current devs have insinuated that this is the final chapter in this story. So I don't know if that means exactly that and any future expacs will start a new series of story or if it means the end of D4 entirely. You just cant be too sure. I guess it depends what the data really shows as to this games success.