Game Reserve Driving
Before going to any reserve, it is important to remember that the driver is just there as an observer, not a participant. Keep your distance from the animals, your speed down, and refrain from having music blaring out of speakers (that has been known).  Most reserves offer good advice on road conditions, the best routes if you are determined to see the big 5 or standard animal hides, many also have petrol pumps for you to fill up. As always, checking locally is the best way to confirm availability, suggested routes, and so on. 
Keep your speed down! The sign is NOT because the tortoise is moving too fast. 

4 by 4?

Sometimes essential, but generally not necessary. In Kruger, you don't even have to stray from tarred roads, (this image shows that some animals in Hluhluwe seem not to as well) although you will miss out on most of the best parts of the park if you stay on them. In numerous game-reserve visits, I have only rarely regretted driving a standard car - and then only because, being so low to the ground, it is often harder to see what is going on.

A standard way to see animals is to watch out for a flock of cars - once one driver has stopped to watch nearby animals, other drivers will then stop, and the number of cars will breed as rapidly as the traffic allows.