When sneaking around at night, you need to get by many of the prefects. When you need to get by them, simply crawl under the bookcase to the left of the portrait that leads to the Griffondore common room. It leads through a secret passage that takes you to the fourth floor. When sneaking around and you do not want to walk slowly, you can use various objects and spells to distract the prefects. Stink Pellets are not too reliable, but the non-explodable luminous balloons work well. As for spells, try Flapendo.
