I don't really know, so I found this online:
"The fourth and final puzzle contains to be a Omanyte which can also be the first puzzle that you can solve. To get access to the puzzle you must get through the Union Cave while getting back from the third puzzle which you must go west after surfing and using HM04 Strength on a rock which will bring you to the final puzzle. After solving the puzzle it unlocks the following Unown: K L M N O P Q."
Hope this helps :)