Ok , here's the deal: Prior to my visit to the Big Island I did A LOT of research wether to drive up Mauna Kea in a 4x4 or a regular rental. For all of you that are asking the same question, let me tell you this: We did it in a two-wheel drive Nissan Altima and we where perfectly fine! There is an unpaved road for less than 5 miles between the visitor center and the summit.
Although the weather in the valley was very rainy the road itself was dry. Once you're above the clouds there is was no problem at all. Maybe we where just lucky but I don't see the point in renting a 4x4 for double the price. However I cannot advise to do as we did. The drive up a regular car worked out for us - it might not for you.
Whichever way of acending you choose, Mauna Kea is worth it. Awesome views and maybe the most beautiful sunset I've ever seen.
At last, two more things:
1. bring warm clothes and GLOVES!!!
2. if you can choose, don't go during full moon. The moon is too bright and you can't see as many stars.
Anyway, have a great trip!