I just turned in a Chevy Impala yesterday morning, putting about 320 miles over 4 days. We went all the way up to Ke'e Beach and to the last overlook in Waimea Canyon without the slightest hint of anything needing 4WD - regular US highways.
I believe that reaching the beach at the far southwest of the island is the only need for 4WD - we ran out of time to get there - but from what I hear it's very passable in a regular car (just quite bumpy) except when it rains.