Enter the castle, then go up the first staircase and through the door with a keyhole. Turn around and go back to the room you were just in, then go in the room with Course 4. Walk down the ramp on the right, and you'll hear the screech of a Big Boo! (What really happened here is that you triggered the Big Boo in the hallway leading out to the courtyard. If you go in that hallway, he won't be there.)