You can still liberate towers and outposts, take on side missions, go hunting, engage in the random events that show up, and generally cause havoc. However, the rest of the world is still open.

When a player joins your game, the main campaign missions become locked out.

It took me a bit of time to wrap my head around Far Cry 4’s co-op mode. The second player is there because (ideally) you both feel like playing around in an open-world FPS sandbox. There’s the potential for the second player to bring back with him or her guns he or she hasn’t unlocked yet, but that’s not the reason he or she is there. Any such progress is only saved to the host’s account, so the second player would have to liberate or collect all those in his or her own game. What’s excluded from that list is any sort of progress within the game, i.e., towers liberated, outposts conquered, or collectibles gathered. This player brings with them all of the skills and equipment they’ve unlocked in their game, and gets to take back with them any experience, guns, skills, ammo, and cash they earn along the way. A second player can be added to your game at any time to play as the free-spirited Hurk, who appears briefly in the game to essentially say “you can play co-op now!” before going back into the shadows.

Far cry 4 coop crack