Gravel road, large potholes, no 4WD required but be careful to not scratch the underbody of your car.
Háifoss Waterfall is probably the most stunning waterfall we have been to in Iceland and we highly recommend to hike down to the gorge instead of staying at the viewing platform. When you walk to the viewing platfrom continue along the path until you find a gate with a ladder to climb over. This is where the track down into the canyon starts. Just follow that track and always have your eyes on the waterfall, you cannot miss it. It takes about 1 hour to hike down and a little bit more to hike back up.