You will need it.
In order to get the full Crete experience, you'll definitely want to have a 4x4 car. There are two reasons for this:
Firstly, the roads can be tricky, especially in the south, where the road network is underdeveloped compared to the North. Also, the south is much more mountainous, which means that there is going to be a lot of uphill roads, many of which are rugged, rocky or dirt roads. Having a sturdy 4x4 car will give you the confidence you need to drive comfortably in those roads without the fear of breaking the car and getting stranded.
The second reason for this is that you can't always trust Google Maps in Crete. My experience is that again, in the south, navigation systems aren't always reliable. For the most part, they are. But getting lost in some random remote village in the middle of nowhere and asking the locals how to get to your destination is something you should be prepared to do. So you better have a good car, because good asphalt roads aren't guaranteed in remote areas. However, it's part of your Cretan experience so you should embrace it! It might even be a highlight to your trip.
Go check out my guide for the best free things to do as well as itineraries and travel tips to make your trip unforgettable.