Welcome to Flyertalk!

First of all, what are your interests? There are plenty of places my kids would love to go that would be miserable for me, so part of deciding where to vacation is finding somewhere with something for everyone. Do you enjoy amusement parks, the outdoors, remote/exotic locations, big cities?

From South Africa you could always do a self-drive or guided safari (depending on your budget), visit the Seychelles (if looking for a beach holiday), venture to Tasmania/New Zealand. Without knowing your interests it will be difficult/impossible to give a relevant suggestion.

In regards to what airlines are good with kids, the Asian airlines are known for being best with children, even in economy (Cathay, Singapore, Thai, Eva, etc.) European/Middle Eastern carriers depends more on the crew, but you should honestly just expect to take care of your child yourself, as they're not there to be babysitters.

As for traveling with a 4 year old, I think that's a great age to travel. We typically go on one vacation each month with our 4 and 2 year olds, so its certainly doable, especially with only one child.