Though it shipped running at 900p, the 1080p patch for Assassin's Creed IV: Black Flag is now available for the PlayStation 4.
"Finishing the game at 900p gave the development team the extra room in terms of the GPU and CPU usage to ensure the framerate never dips below 30 frames per second," explained Ubi.

Though they also say that you probably won't even notice the difference between 900p and 1080p. Now, if it were a jump from 720p to 1080p, then yeah, you'd probably notice.

They also said they've added a "brand-new anti-aliasing technique" to the game.