I'm playing Ultra Sun Right now, and I saw something that looked like a fifth island. It is small and round, and it's in the northeast corner of the map of Alola.
I've beat the e4 3 times and finishedd RR episode. It's not aether paradise, is it an event-only area or what? I want to get to it!
You can even see it in all of the official maps of Alola!
Bulbapedia -> Alola
To simplify, Can I get to this Island, and if so, how do I?
It is speculated, with good evidence and reason, that the northeast island is Poke Pelago. In the official artwork, you can see a small part of a beanstalk, behind a cloud.
Hope I helped!