I'm yet to play Unity, so I don't know how much it shakes the previous AC gameplay. But from basically 2 from Black Flag the game feels the same when you're on foot. It's good, because five minutes in and you're already in the gameplay loop, but it's bad because it's mostly the same clumsy thing it always was. It's all fine and dandy until you're desperate for something and you see your movements becoming erratic because of the free running. It's not a dealbreaker, but sometimes it's annoying. Also annoying is the gigantic amount of tail missions in Black Flag. Some are fine, but some are infuriating. But those are, in a sense, nitpicks, because that's the most fun I've ever had with AC (I played 1,2,3 and now Black Flag. I skipped the two other games of the Ezio trilogy but I have them and I'll tackle them some time in the future. I was just too eager to play 4).
For what I've see in videos, the core gameplay only changes from Origins\ Odissey, which are two games I'm curious about, because I love both pharaonic Egypt and classic Greece, although both games seems to have some serious microtransactions in there, which is always a bummer. Still, Origins and Odissey are regularly in PSN sales, normally very cheap, so I'll get them both eventually xD