Make your way back to the Temple of the Ocean King. The symbol is the same, but here's the catch: when you get to the bottom (where you found the 3rd Sea Chart) go back to the previous chamber and put the crystals back in this order: square, circle, triangle. Then go back and the floor shall sink. You've just found the last few floors of the Temple. Now, fight through the last few floors and claim the fourth and final Sea Chart. The seas are now yours to tame!
Also:
There is a rather extensive sidequest which can be very rewarding. Go to the northern Traveler's Ship, defeat the monsters, and the Man of Smiles will appear. When he asks you what you want, say "something mysterious." You'll get the Hero's New Clothes (though you can't use them). Now begins a trading game. Take the clothes to the so-called hero in the NW quadrent and give them to him. He'll give you a telescope. Take that to the Ho hos in the SEQ and give it to the telescopeless Ho ho. He'll give you a Gaurd Notebook. Take that to Nyave in the SWQ and give it to him. He'll give you the Wood Heart. Take that to the new ship in the SEQ and give it to...the Old Wayfarer! He'll hint he has something good for you back at his hut. Make your way to Bannan Island and...fight another giant purple thing. Once it's defeated, go to the Old Wayfarer's hut and get the Swordman's Scroll! Now you know the Great Spin Attack!
How was that?