What HP's do people feel require a 4x4 vehicle to reach the trail head? I did all the high points above in my own 2 wheel drive mid-size vehicle in the last two years without any problems.
The worst HP road I've yet driven has to be Mount Arvon's logging roads. This was accomplished in the same vehicle, but I could definitely see the need for a high clearance 4x4 if it had rained hard recently.
I haven't done it yet, but I understand you're supposed to rent a 4x4 for the drive up Mauna Kea.
Scanning old trip reports, I read complaints about the roads to Mt Elbert and Mt Sunflower. Mt Elbert's gravel roads were being re-graded when I was there two summers ago, and Mt Sunflower must have had similar treatment sometime before that.