I know it may not sound real but it is true! There is a 4th Dark Realm.

The 4th Dark Realm is inside the Oni Mansion. Once you are inside the mansion reach the end of the maze.(Beside the protective mirror.) Hit with any of your weapons the pink screen. Enter the small passageway where on your left is a hole and on your right is a bundle of boxes.

DO NOT ENTER THE HOLE!

Keep pressing the X button on the boxes. After whenabout 2-5 seconds the man, wrapped in the green material who hangs from the ceiling, shall appear. If you press the X button on him he shall ask you if you would like to go to the Dark Realm.

Note:

1. I have only tried this on Hard Mode.
2. The reward should help you in the next battle!
3. Obviously the Oni Mansion is only available when playing as Jacques.