"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."