From a time and distance point of view it is easily doable. I've done JNB - Crocodile Bridge (468 km) in 3h50m, at an average of 127 kph. That was out of necessity, and before the N4 became a toll road and freeway. Not recommended.
The back roads are by definition slower, but often far more scenic.
In South Africa you drive on the left, and you need to get used to the traffic.
The affluent minority drive luxury sedans and SUVs, and many of them seem to think that 160+ kph is the speed they are entitled to drive at, the rest be damned.
Then you get those who can barely afford a vehicle. They can certainly not afford to replace tires or maintain their cars, and usually drive at 60 - 80 kph, trying hard to stay on the road.
There is a lot of heavy traffic, with trucks and semis spanning the whole range from spanking new to wrecks with all the wheels pointing in different directions.
It is also pretty safe to assume that about half the drivers bought their driver's license at the local fish market, and DUI is endemic.
On top of all that, you get pedestrians, often in large numbers, walking along or crossing roads, including freeways. At night, many of these are drunk.
Add in the cattle and goats in rural areas, and you get the picture. Last year they had 1465 fatalities over Christmas (no doubt a significant undercount), it will be interesting to see whether they set a new record this year.
As long as you keep all this at the back of your mind and adjust your behavior accordingly, you should be fine.
Johan