The naval combat and exploration is the high light of Assassin's Creed 4. It's so well done and polished, that alone would be reason enough for any fan of open world games, to try this game. The world is massive, is beautiful and relentless. The weather effects look fantastic, to the point that starting a trip to the open sea, at night, under a sea storm, can be a scary experience!
The exploration factor is very polished and well done in this game, but could be alot more exciting if you couldn't see every single pickable item on your map. Series like The Elder Scrolls perfected the exploration factor, since you enter a new region or dungeon, and you never know what you may find there. Maybe just some coins, maybe a powerful artifact or weapon.
The map is huge and is very well designed too, but it would be cool if there weren't invisible walls. Sure, this is improvement over previous games in the series, but there are still many places in Assassin's Creed 4's islands, that you can't visit.
The fact you also can manage your fleet through a turn based strategy game is a very interesting addiction, and another 'little' thing that makes the world feels even bigger and alive.
The Pirates story and characters are very well done too, but as always, the modern day/Abstergo story kills part of the experience to me.
The graphics and performance are really good on PS4! I never noticed any worth mentioning slowdown, and the native 1080p looks as clear as ever.
Since i'm not a fan of Assassin's Creed, i would love Ubisoft to develop that Pirates game completly disconnected to these series. No modern day anticlimax interruptions from Abstergo, and a toned down free run/parkour gameplay (it creates as many cool situations, as it creates frustrating ones).
Overall it's a fantastic game, and most of my complains are personal ones. I would really like to see that Pirates game, probably in first person, more close to a slower paced RPG, than an action adventure/parkour game.