If I had 4 weeks I would focus on Palawan and the mountains of north Luzon, which gives you good variety... Palawan has the most spectacular marine scenery, and the mountains make a good contrast. I would avoid the island-hopping thing, which tends to drag you from one sandy beach with coconut trees to another to another, spending a lot of time on boats and buses to get to places that aren't much different from the last place. 4 weeks gives you time to do Palawan and the mountains thoroughly, and to add one more specific destination if there's something in particular that you want to do.