Only 3 planes are needed.

Only 3 planes are needed. Planes A, B, and C leave the island together. After flying 1/8 of the distance, plane C transfers 1/4 tank to A and 1/4 tank to B. This leaves C with 1/4 tank, which is just enough to go back to the island. Planes A and B continue another 1/8 of the way. B then transfers 1/4 tank to A. B now has 1/2 tank left, which is just enough to go back to the island. Plane A now has a full tank and continues to fly until it runs out of fuel 1/4 of the way from the island. It is met by C which has already been refueled. C transfers 1/4 tank to A and they both head home. Planes A and C run out of fuel 1/8 of the way from the island. They are met by Plane B which has now been refueled. B transfers 1/4 tank to each of the 2 planes, and the 3 planes now have just enough fuel to get them all home.