UC4 will be a beast for sure.
First-of-all, I've just come out of a long open-world rambling, but in this case, comparing AC:U to UC4 is not correct if it's PC vs PS4 (AC:U runs at 900p at 20 FPS on console, come on!) and it's open-world vs linear game. Naughty Dog will come out on top in comparisons when the game is done, end of story.
What is obvious is that, as is known, they use the same engine for UC4 as for previous-gen. There's this core which is visible under all the polish and smoothness which is still old-gen (like grenade not causing and ground deformation or lasting impact).
Now it's important to also understand that this time we get a real run-of-the-mill UC gameplay for a demo and not a staged scene, like for UC2. This is what this game should look like at it's "worst" in my opinion: pre-alpha, not set-piece, no gimmicks. Just a jungle, couple of soldiers and platforming. So far: must have. Maybe won't beat the Order, but that game is all set-piece and is cropped on both sides.
2015 will be great for PS4.