I'll try to help you understand the Island Rating since it can be confusing to figure out. These are the Statements that Isabelle says when you have a 4 star rating:
"Lacking appealing scenery" it means that you don't have enough furniture decorated your island and you don't have enough fencing. Do this make sure you put a lot of furniture all over the island and have more fencing usually I just use fences to make big gardens.
"Landscape has potential" it means that you need to have variety of trees planted (Saplings, Cedar Saplings, Fruit Trees) and a lot and I mean A LOT of flowers planted on the island. This one is hardest one because you really need to plant so many flowers just so that Isabelle is more satisficed and the rating should go up.
Hope this helps you.