Instructions
Go along Route 32 and enter the Union Cave.
Go up the path and descend the stairs.
Use your "Surf" skill to cross the water and head to the top of this area.
Go along the path and turn left at the top before turning right at the fork.
Ascend the stairs and use your "Surf" skill to cross the water and ascend another set of stairs. Go through the door and head into the derelict building, going to the bottom right side.
Head up to the next derelict building and go left; you will find the Fourth Chamber that contains the Ho-Oh Puzzle.