It's less that it's outdated but more that they've just been making small iterative changes since Brotherhood. Not only that, but things that are updates in one game don't seem to make the jump to the next, kinda like what happens with the COD games.
For example, Revelations had a couple things going for it that never made it back into the series. Like, when you're playing a story mission where you are walking with an ally to a location, you can make it so your character just auto-follows the ally, so you can sit back and just listen to the story instead of being annoyed about having to hold Up on the joystick and walk slowly with them. That never made it into 3/4 and i thought it was actually a good way of handling that.
Also, Revelations had some amazing uncharted-like platforming sections that were the real highlight of that game. Stuff like that never made it into 3/4, and that sucks because those scenes were really epic and added a lot to you feeling like a badass. Meh.