I traveled this road twice , last time about 6 months ago, you do not need 4x4, just a car with higher clearence, there are some rocky bits, especially when going down to constanza, so if you have low clearence car, yoy will have to be carefull, you will hit the bottom here and there, but it is not impossible.
Also on the way you will pass a Piramid, which is supposed to be the geographical center of the island (if the info which was given to me is correct)