4WD or AWD to visit Mauna Kea?

[Kelena]

I guess I really don't know the difference between a four wheel drive (4WD) vehicle and one with all wheel drive (ADW). All of the signs at the visitor's center say that 4WD is required and I assume they mean it to the exclusion of AWD vehicles (although I have seen some cars at the summit that I am pretty sure do not have 4WD). Some cars, such as certain Subarus have ADW.

Is there a difference?