Slated for October, according to Ubisoft site
Although Assassin's Creed IV: Black Flag has been acknowledged by Ubisoft, the full reveal has yet to happen. There's currently a teaser site set up for the game, and through tinkering with the URL, Game Everything was able to find out additional information that hasn't been announced yet. Namely, Black Flag is due out on October 29, 2013.
The page has since been hidden, but not before screencaps were taken. The Ubisoft site also mentioned that the title will be "coming to next-gen consoles" in addition to the confirmed Wii U, Xbox 360, PlayStation 3, and PC versions. These guys can't catch a break!
Assassin’s Creed IV: Black Flag dated, next-gen versions confirmed, more [Game Everything via Gematsu]