How to reach Island #4 (Floe Island) Pokemon Leaf Green & Fire Red
In order for you to gain access to Island #4 (Floe Island) is to beat The Elite Four for the first time and save your game. After loading your saved game file you'll find yourself in Pallet Town; Professor Oak will approach you and bring you into his lab before giving you an updated version of the Pokedex.
After you go through these scenes and are free to play your character again, you'll want to travel to Vermillion City and take the boat in the southern end of the town to Island One. Once you're on Island One speak with Celio in the Pokecenter (the same guy you spoke to the last time we were here, in the screen shot above).
Now that you're on Island 1 and have spoken to Celio travel back to Mt. Ember (it's the place you most likely went to earlier in the game and caught Moltres). If you didn't go here earlier in the game, you can find this location to the North of where we got HM06 Rock Smash.
Once you reach the mountain, travel northeast until you come across the two Team Rocket members (shown in the screen shot above) they'll be talking about the Rocket Warehouse passwords, and say one of the passwords outloud - shown in my screen shot above. Defeat them and enter the cave that they were protecting.
Travel through the cave until you come across a room with a Red Gem in the center of it. What you're looking for is shown in my screen shot below. Pick that up and bring it back to Celio to gain access to Island #4.