Instructions
Defeat the Elite Four with a team of six strong Pokemon. Form a team of different type Pokemon (for example, use one ground-, grass-, fire-, psychic-, water- and electric-type Pokemon) so you have the best chance of defeating each member of the Elite Four.
Visit Professor Oak's laboratory after your game restarts after you defeat the Elite Four. Talk to Professor Oak to get the National Pokedex. You will need to have captured 60 different Pokemon to get the National Pokedex.
Travel west to Vermillion City and go to the pier. Show the ferryman the ticket you received from Bill allowing you to visit "One Island." Travel to One Island.
Travel north on One Island until you reach the mountains. The cave door on the mountains is now open (there were Team Rocket members blocking it before you defeated the Elite Four). Travel through the cave until you reach a room full of glass squares with braille writings.
Go down the stairs in the room with the braille to access a room with a ruby slate. Pick up the ruby slate and use an escape rope to leave the cave quickly. Travel back to the Pokemon center on One Island. Deliver the ruby slate to Bill to receive tickets to Four Island.
Speak to the ferry operator located on the south side of One Island. Show him your new ticket and select the "Four Island" option on the list he gives you.