.. Opening the fourth floor window

You must've found the hidden room and noticed the wiring goes to the roof?

You would need to do something in the hidden room to get forward.

Look at the objects you have there. ( There's skull on the shelf. There's a crawl space.)

Take the skull from the shelf and drop it into the crawl space.

In the children section. The librarian Doris is also there and gives you the floor key.