The subject of motor vehicles was covered recently (a couple of months ago).
In short. No, you do not need a 4WD unless you are going to leave the bitumen, and you can drive around (and up the middle, and from east to west) the whole country without leaving bitumen.
I have even driven half way around the country with my wife and my dog in a Mazda 121 if that makes the point any clearer.
Regards,
OpsN.