Four Island

Walkthrough
With the Main Story completed and the National Pokedex obtained, head back to One Island and defeat the Team Rocket Members (who are still causing trouble despite Giovanni having left the organization) blocking the entrance to Mt. Ember. After you do, you can enter the mountain in order to find the Ruby. Bring the Ruby to Celio to obtain the Rainbow Pass, the same that will allow you to travel to Four Island, Five Island, Six Island, and Seven Island. Four Island is a very small island with not much more to it outside of the second Pokemon Daycare and Icefall Cave. You will run into your Rival when you first arrive on Four Island, but he will not challenge you to a battle.

Move Tutors
Body Slam: Move Tutor Can Be Found In One Of The Houses

Wild Pokemon
Surfing:
Wooper(FireRed)/Marill(LeafGreen) (70%): Levels 5-25
Psyduck(FireRed)/Slowpoke(LeafGreen) (30%): Levels 5-35

Old Rod:
Magikarp (100%): Level 5

Good Rod:
Poliwag (60%): Levels 5-15
Goldeen (20%): Levels 5-15
Magikarp (20%): Levels 5-15

Super Rod:
Poliwag (40%): Levels 15-25
Poliwhirl (40%): Levels 20-30
Gyarados (15%): Levels 15-25
Psyduck(FireRed)/Slowpoke(LeafGreen) (5%): Levels 15-35

Items
Star Piece: On Ground
Stardust: On Ground
Ultra Ball: ItemFinder

Poke Mart Items
Ultra Ball, Full Restore, Max Potion, Revive
Ice Heal, Full Heal, Escape Rope, Max Repel