Yes, Ubisoft is still using their version of the online pass, dubbed uPlay Passport, on Assassin's Creed IV despite the immense dislike for the practice -- even EA doesn't do that anymore. But now the Passport isn't merely locking online features only.
Game Informer has noted that if you don't have or don't enter a uPlay Passport code that is bundled with new copies of the game, you will not have the option to add a ship to your fleet when you take it. Thus, you can't send them off on automated missions to earn some extra loot as you play. When you do this, you have the option of having friends help you move the missions along, but you can do them completely solo if you wanted to. That's not the worst thing I've ever heard of, but it does make borrowing the game from your buddy a little less of a promising concept, which I assume was the point of doing this -- online passes are there to encourage you to buy your own new copy of the game.
If you're still waffling on AC4, I have what you need: 20 minutes of Dave playing the story.