Hi,
Re: Discount Hawaii Car Rental - we are also satisfied customers. We've booked cars on 3 islands for our trip this summer (G-d willing & the creek don't rise).
Please make sure, though, that you get the LOCAL number of the agency where you pick up your car (with us that was Thrifty RAC in Kona). We went snorkeling with the car key in my husband's swim trunks pocket ... yeah, I know, dumb mistake. When our car wouldn't start because the key's electronics had gotten wet, we went through at least an hour of agony in the Keauhou beach pkg lot because we did not have that number. Had to call 800 #s on the mainland, deal with twits in Oklahoma who told us we would have to pay $450 (!) and refused to give us the local number. I don't remember how we found it, finally (I believe it was through AAA), but we had to have our car towed back to the Kona airport and replaced. Since there seems to be exactly one tow truck in the Kona area, its driver is rather busy and you will have to wait as we did (didn't help, either, that the driver was a new arrival from Fresno and was as dyslexic as we were in pronouncing HI street names).
As far as 4x4 goes -- we had a Jeep Commander which held everything and was quite secure. We took the Saddle Rd to Mauna Kea -- "everybody does it," we were told (we are experienced off-roaders) and frankly that road is in no way, shape, or form a problem. The stretch from the Onizuka Visitor Center to the very top is mostly paved ... no one tells you that. We have had to negotiate far more tricky roads in rural California. If you're from NYC, say, don't drive usually, and have never gone offroad, maybe you should think twice. But don't buy the anti-Saddle Road propaganda. It's easy and nobody cares.
Have fun - we loved it.