You have to beat the elite four first and go to the sailor in vermilion city and go to one island for the rainbow pass from bill. then you can go to four island and the rest with the rainbow pass.
Only in firered and leafgreen can you venture to the sevii islands and four island.
Bill asks you to go to one island not four island and second of all if you finish the 1-3 island sidequest and get the ruby and deliver it to Celio you can go to islands 4-7.
go to One Island
beat the elite four then go back to island one and talk to Celio in the Pokemon Center
go to four island. beat team rocket in four island. go to six island. use cut. the door will open. get the saphire, but a guy steals it from you, and gives you a second password. go o five isaland. the doors will open with two passwords.
you have to beat the elite four to get to four island. talk to celio after beating the elite four. he will ask you to find him a ruby. the ruby is located on 1 island in mount ember. find it and give it to celio. then, he will give you the rainbow pass, which allows you to go to four, five, six, and seven island.
Go to one island and go to mt ember go inside and find the ruby and give it to celio who is in the Pokemon center on one island *note* you must have beaten the elite four and got the national dex to get the ruby and strength.
go to the ship station (it's how u got to four island) and talk to the dude. he will ask u what island u want to go to. choose the 1 u want.
u need rainbow pass then talk to sailor
One island.
go get mewtwo, the legendary dogs go to 1,2,3,4,5,6,7 island and have fun
You need to get the ruby from mt ember then deliver it to celio.