Assassin’s Creed IV: Black Flag has a “narrative connection” to another Ubisoft title hidden in the game, according to Ubisoft’s Ash Ismail.
Of course Ismail wouldn’t reveal any specifics about what the link is or which game is involved, but he did say that fans would be pleased. There are two real possibilities, the first is a link with Far Cry, and considering that ACIV is set on a Caribbean island it makes a lot of sense. The second is Watch_Dogs, which could easily be linked with the present day sections of the game.
The link could be absolutely anything from a mission to a simple word from a NPC, so don’t go expecting too much.