I rented a small 4x4 and it was fine. During wet season the roads can get washed out so be careful. In addition, the roads can be filled with potholes (think moonscape) so take your time. I didn't use the 4 wheel feature, but it was great to have the extra ground clearance of a 4x4 for those holes. Driving in Costa Rica is really not that bad. I drove from San Jose to Mt. Arenal and then to Manuel Antonio.