Since you are there for 4 weeks, why dont' you consider going to Phi Phi island, Koh Lanta and Krabi (Railay, Ao Nang). all are easily accessible from Phuket.

Been to krabi 3 times and I like it for being being more quiet and peaceful from the nice and action of Phuket esp the patong area.

Picture of poda island which is just off AoNang


View of railay beach from the cliffs



In any case enjoy your trip