Upvote:3
OpenStreetMap has the 606 into Santa Elena from the north paved, which it wasn't when I was there, also coming from Arenal. The 619 still seems to be unpaved along with some of the smaller orads. The unpaved roads in that area would have been tricky in a normal car in dry weather, more for ground clearance than anything else, with the transition from paved to dirt being particularly prone to big potholes. In the wet you'd need the grip of 4WD. Around Arenal we were mostly on paved roads but I recall dirt roads, perhaps into the reserve. The Puntarenas/Nicoya region had a lot of dirt roads too.
The rental price for a compact SUV (we had a Rav4) was reasonable, and availability was good.
Some of the wildlife hotels have quite long dirt access roads, and while they might be doable in a standard car they'd get pretty tedious if you're going out once or twice a day for a few days. Generally if you're on a wildlife quest, you'd want to be prepared to go to areas only reached by rougher roads.
Based on my experience, you'd enjoy it more with a light 4WD than a more conventional car, even in dry weather. In the wet you'd benefit more from the 4WD. On the worst stretches you might benefit from doing a little homework as it could get tricky if you're not used to it.