To ease the transition into the next-gen, both PS4 and Xbox One offer ways of upgrading cross-gen games on the cheap. And while games like Battlefield 4 and Call of Duty: Ghosts will allow players to transfer their multiplayer stats between generations, single-player data will be lost when moving between generations.
A Kotaku report specifies that a number of fall releases will not allow players to import their single-player save progress. These titles include Battlefield 4, Call of Duty: Ghosts, and Assassin's Creed 4. Considering that the latter is largely built on a single-player experience, the lack of cross-gen save files might be an important detail to consider.