At the very start of Chapter 4, climb the rocks and then head down the hill on the right hand side, instead of continuing left, and you will notice a man attempting suicide. Use big brother to lift the man up, and then climb the tree and undo the rope using little brother. After saving the man, you'll need to use the path behind the tree to get into the burned house, save the music box and deliver it to him. Upon giving him the music box, the achievement will unlock.
This achievement can be seen in the video guide at 4:36