Assassin's Creed 4 Black Flag: guide to defeat the 4 legendary ships
Our career as great pirates of the 7 seas also passes through the clash with 4 powerful ships: the Legendary ships.
Caribbean Devil Guide: Defeat the 4 Legendary ships
The Legendary ships are located in the 4 corners of the game map. In one of these locations you will find two sister ships, you will have to destroy them both but they are counted as one ship. Although it is not necessary (you can attack ships at any time) before embarking on this quest it is advisable to fully upgrade your Jackdaw [guide] ship, which you must do if you want to unlock the "Naval Engineer" achievement as well. This will make it much easier to defeat the other ships.
Destroying the legendary ships, in addition to making you unlock the "Caribbean Devil" Trophy / Objective, will also earn you a lot of money: each of the 4 battles will in fact yield you well 20.000 Real. Not only that, after defeating the 4 ships, an ability is added to the jackdaw, the retrocarica, which allows you to ram with much more power.
In the following list you can see what are the legendary ships, while the video shows good strategies to face the fight and win. Consult the Assassin's Creed 4 Black Flag cheats and guides tab for other useful guides on this game. Good luck.
The 4 Legendary Ships
1. La Dama Negra (South West)
2. HMS Prince (South East)
3. El Impoluto (North West)
4. Royal Sovereign & HMS Fearless (North East)
Thanks to SMC93 for helping to improve the guide in the comments.