I'm guessing your talking about four island five island six island and seven island.
First, you have to have been to 1,2,and 3 islands.
After beating the pokemon league go to one island and talk to ceilio. ( Bill's friend ). He will ask you to get him a ruby. The ruby is at Mt. Ember. Then, he will ask you to get him a sapphire, its in the dotted hole.
To get him to talk about the ruby and sapphire though, you have to have the national dex which you receive by owning 60 different types of pokemon. Then after you get that many types of pokemon owned on your pokedex, beat the elite four again, and professor oak will give you the national dex. hope this helped :D
After that he will give you the rainbow pass, allowing you access to 4-7 islands.
Here are maps for the dotted hole and Mt Ember