We were there in July....had a conventional car. I would consider a 4-wheel drive if I were to go again. However, the only place I might have gone - that I didn't go this time - would be the green sand beach.
We saw Waipio also via the shuttle van that took us to horseback riding.
Best advice....get the book, Hawaii: The Big Island Revealed. It is very specific on what attractions need a 4-wheel drive.