A 4WD is only needed for reaching the summit of Mauna Kea (which you shouldn't be doing unless you're an experienced 4WD driver in steep, extreme conditions and can handle the altitude), driving to the Green Sands beach (which you shouldn't be doing because none of the major rental car companies have 4WD cars appropriate for that drive), or driving into Waipio Valley (which you shouldn't be doing unless you've personally assessed your vehicle's ability to handle extreme steep terrain and crossing water).