On the fourth floor there are bookcases (4) go to the second one nearest to you, it should say on the a button. push when you push it it should open. (Dont push the hogwarts button or it will send you back to the other side of the bookcase. Follow the tracks and you should see a chest it is covered in cobwebs, use the fire spell to open it then it will open but dont let it fool you because when it is open he wont get the card, unless you press the A button to search At the end there should be 2 doors but they are locked so look at the bottom of the walls there should be an empty space there, crawl through and you should be at another floor
Happy playing