Even though it's technically the completion of the storyline they started I believe, I would skip Assassin's Creed 3. The setting is kinda cool because it's not a very common one we've seen in games, but I thought the game was generally garbage. The story is awful, most of the characters aren't good, especially the lead. I got bored with the first game and never played the others up till AC3, but I hated that story and the exploration in it can get incredibly dull.
AC4 on the other hand, though I need to go back to it and actually finish it, it is just so much better. Much less of the dumb modern stuff, really awesome pirate setting and exploration, the ships are awesome, that's the only one I can recommend.