Act IVAct IV starts with you arriving on the Banoi Maximum Security Prison.
You eventually get to a room where Titus asks you to find another Armory the second time. Ryder White contacts you and tells you not to arm the prisoners. He then asks you to go through the sewers. Go to an elevator and encounter another prisoner who has helped you in a previous quest. He tells you that Ryder is not going to get off the island and make more, but that he's going to drop a nuke on Banoi Island. You then run to the top of the prison and have a gun fight with various soldiers working with Ryder.
Emily bites Ryder and then Ryder shoots Jin twice, causing her corpse to fall off the helipad.
Ryder uses the vaccination and turns into a zombie that incorporates most of the zombie's abilities that you have met in the game. You are forced to kill him. Once you kill him, you escape Banoi in Ryder's helicopter with one of the prisoners. Logan, disbelieving that he's finally made it off the island, says he can't wait to get all of his old problems back and for everything to be the same. Kevin disagrees, saying that things won't be the same and never will be.