Things You'll Need
Instructions
Start looking for the Broken Butterfly at the start of Chapter Four. From the chapter's beginning, go west with Ashley through the hall where you'll see the Merchant. Pass right on by the Merchant, you will not be needing any of his wares this time.
Continue going west through the next room until you come to the room with the dropped cage. The area you conquered earlier will have refilled with the "infected". You can kill them again if you want or you can simply try to avoid them.
Run through the the doorway to the south trying to avoid the regenerated infected. Exit back to the halls just south of the hedge maze.
Don't move. As soon as you enter the area stay perfectly still. There is a large insect clinging to the ceiling just south of the doorway and another to the west around a small corner. These insects can be killed quickly with your rifle if you can do it before they have a chance to spot you. Once they're dead, head out to the main hall.
Check your map. It should show a locked door to the north. Take Ashley there with you. She'll use you to piggyback over the wall to unlock the door for you from the inside.
Pick up all the free stuff. There are a lot of useful items to collect in this room. Now that you're there, don't forget to grab your free Broken Butterfly. You'll find it in a sealed chest.