How do you get to four island in Pokemon FireRed?

Answer 1

You must get a mystic ticket by going to a poke-mart and answering the questionnaire the answer for the questionnaire is "Link Together With All". Then you must download the mystic ticket with a wireless adapter.

I thought four island was included in the rainbow pass (Dzhx)

Answer 2

You must do several things to reach Four Island: First get inducted into the Hall of Fame, Second obtain the National Pokedex from Professor Oak which he only gives after seeing 60 pokemon recorded in the pokedex, Third go to Three Island and save a girl named Lostelle in Berry Forest (if you've done this skip this), Lastly go to One Island and go north to Mt Ember and talk to the Rockets to the right and they will battle you. Win and go through the cave behind them and use Strength to move the boulders away until you reach a interesection where you can go down or right. Choose down and you will eventually reach a room with a red jewel, pick it up with the A button and you will obtain the Ruby. Go to the One Island pokemon center and talk to the guy with glasses near the large machine to the right of the center and he will give you a Rainbow Pass which will allow access to islands 4-7.