At Big Island Jeep Rental, we focus on 4X4 rentals. This is because here on the Big Island, the land, roads, driveways, and even parking lots can be pretty rough. This is what makes the Big Island cool - we aren't really that overdeveloped.
Even if you don't get a Jeep, we suggest getting something higher off the ground.
Below is one of the many "do you need a 4x4 Jeep in Hawaii?" videos we shot a while back. As you can see in this video, a 4X4 is definitely handy.
For the big one- Mauna Kea - you need a 4X4 with a true 4-low transfer case to help slow you down so you don't ride the brakes. Not all 4x4s have this 4-low transfer case. In fact most 4X4s you rent are more like all wheel drive.