I would say you can spend one hour walking around the main village, one hour by hiking up and down to the viewpoint and another one for having lunch. Add another one for sunbathing and swimming and the 4 hours are over. You could also think about taking a longtail boat out of Ton Sai Bay to Phi Phi Le (Maya Beach "The Beach") if it gets too boring on the main island.

Regarding public showers. Not sure if there are any available but i would ask at any of the local dive shops or hotels if you could use their facilities (pool area) just for a quick shower.