In our hands-on time with Far Cry 4, Ubisoft elaborated on how those without the game on PS4 will be able to take part in two-player co-op with those that do.
Single-player campaign missions are separate from two-player ones. Four player co-op found in Far Cry 3 will not be found inside 4. Because the two-player missions are separate, Ubisoft has developed a unique game download that contains only the assets from the two-player experiences. It will be substantially smaller than the game itself.
The second players will control Far Cry 3‘s Hurk.
Far Cry 4 will be available on Xbox 360, Xbox One, PS3, PS4 and Windows PC. The freeplay features mentioned about are only available on the PS4.