Of course there are morning and evening rush hours, and these cause traffic to back up somewhat, particularly when you consider that Auckland is built at a pinch point on North Island, so the road system has to take account of limited crossing places over bodies of water. However, while locals, and outsiders who are used to rural surroundings, will moan about Auckland traffic, it is not difficult when compared with e.g. London.
Don't forget that the 4 million (and a few more now) population are not evenly spread across NZ. One and a half million of those people live in Auckland.