Originally Posted by
bocastephen
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).
I've heard the road to the green sand beach has gotten a lot worse since I last did it, but it used to be perfectly doable in a wrangler. Waipio Valley isn't really a big deal *if* you know what you're doing and know how to use 4 Lo (unlike one of the guys I ran into at the bottom that was concerned that his brakes no longer really worked). Going back up feels kinda weird, the angle is so great.
Both of those two I would definitely say shouldn't be attempted unless you know what you're doing in a 4WD (ie, you have experience off-road, not that you own a 4WD that never leaves pavement). And Waipo it needs to be a real 4WD that has 4Lo, I wouldn't attempt it without it.
Mauna Kea summit isn't a big deal, and could be done in a 2WD, but they don't want you to do it without a 4WD because it supposedly tears up the road more without it. I honestly couldn't tell you if that's true or not, but there's nothing real extreme about the road. I probably wouldn't recommend it if you've never done any mountain driving before though.
There's a couple of other places I've been there that I'd rather have the 4WD (or at least something with high clearance), but there's so much to see that you don't need one for that you're not going to lack for things to do without one.