It's pretty easy to figure out which key goes where, really. As soon as you put a key in its matching hole, a piano note sounds. So even going by trial and error, you'll get the answer soon enough. Once all four keys are in their proper place -- the first key you received goes in the fourth hole, the last key you received goes in the third hole, the second key goes in the first hole, and the third key goes in the second hole -- the notes sound again, and continue into a full song.

As it plays, the door swings open just a crack. Will you go inside?