We've driven everywhere (well, almost everywhere) on Kauai and never used a 4WD. The only really bad road was the one to the beach on the West coast, near where the Na Pali cliffs start. It is rutted and bumpy but OK if you go slow. Some have recommended a Subaru AWD. We're renting a Ford PU for our next trip because they are/were on sale thru Budget.

Phil