Verified answer

In order to get the other islands, beat the game once, get 60 pokedex and then speak to professor Oak. He'll upgrade your pokedex to a national dex. Go speak to celio. Then head to the place where the 2 team rocket grunts blocked the way in Mt. Ember, 1 island. Follow the path, get the ruby and go back to celio. He'll then add 2 sheets to the map. Now, you can go to the other four islands.

Oh yes, you can capture johto pokemon in these 4 islands.

Leaf Green Island 4

Do you have your pokedex upgraded to its national form?