To get to it:
1. Enter the Mansion: It's found in the south-west corner of the map, near the village of Celestic Town.
2. Navigate the first floor: Be prepared to solve some puzzles and fight some enemies.
3. Find the staircase: You'll need to find a secret passageway on the first floor, leading to the second floor. This is usually hidden behind a bookcase, but you might need to interact with other objects to make it appear.
4. Find the Song Book: On the second floor, look for a room with a broken window. The fourth Song Book is in this room.
Remember, the fourth Song Book is only available after you find the first three Song Books.
Good luck finding it!