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Survive Episode 4.

Online required. These can be done in private solo matches. As noted in the overview, to survive each of the episodes, you have to get through 2 days in-game, which should take just under half an hour real time. To unlock episode 2, you have to complete any 3 missions in episode 1. To unlock episode 3, complete any 8 missions in episode 2, and to unlock episode 4, complete any 12 in episode 3.

Note that you don't actually have to complete any missions to survive, just get to the safe house at the end of both days.