Far Cry 4's co-op is available to people who don't own Far Cry 4. How is this possible, you ask? It starts with a client.
Like the 3DS's download play, PS4 users without Far Cry 4 will be able to download a separate app that allows them to be invited into their friends' Far Cry 4 games.
It's going to be a colossal app, since it will contain all of the game's content sans its story-based missions - which are the only content available exclusively in single-player.
Technical director Cedric Decelle told Eurogamer that the studio is tweaking co-op play to be more difficult than playing on your lonesome. The developer is still experimenting with what this will effect exactly. It could be hit points, damage received, enemy AI, or any number of factors that are currently being tested.
Furthermore, the second co-op player is Far Cry 3's Hurk, making this the first time a character has appeared in two Far Cry games. Does this mean they take place in the same universe? Or is it a goofy crossover? I believe this is open to interpretation.