Zero Escape: The Nonary Games

To open the fourth lock, first you need to find the IG Replicator by going through the Yellow Door choosing Ally in the first AB Game and then choosing the Red Door. During the escape room you'll find the IG Replicator which you'll discuss in the novel section though you'd have no idea what it's purpose is at that point in time.

To find the fourth lock once you have seen the IG Replicator, you need to go through the Cyan Door, choose Ally in the first AB Game and then you'll automatically go through the Blue Door. As long as you've seen the IG Replicator, in the following novel section you should automatically unlock this achievement.