I can only assume you put "next-gen" in quotes because you're referring to other cross-gen games. Because AC4 is pretty damn weak compared to actual next-gen games that didn't have to cater to PS3/X360 versions. AC4 is a last-gen game through and through, and it looks it, no matter which next-gen system you're running it on.AC4 was an absolutely superb game with gorgeous graphics that can easily stand up to other 'next gen' games.
You're right in that the gameplay is fine, but it is starting to get a bit stale, and it's far from perfect. New hardware opens up new doors in terms of AI, physics, etc, and they'd be idiots not take advantage of it.