My experience with traveling in the morning (any day of the week) has fairly decent; airports in New York seem to be less busy in the morning than in afternoon / evening.
It's only 2 minutes on the AirTrain for every Terminal stop, and it runs every 10 minutes. (Warning: you do have to do a lot of walking within each terminal, though!) You probably won't have to wait more than a couple of minutes - there are three routes, and all three of them stop at every terminal. You'll probably want to take either the Howard Beach route or Jamaica Station route, since those run in a counterclockwise loop, meaning it's one stop from Terminal 3 to Terminal 4 (rather than having to go all the way around the loop on the All Terminals route.) Also, the AirTrain is free between airport terminals.
I don't know anyone who has traveled on a South African passport, but I've never experienced terribly long lines at immigration / passport control. Security lines can be the most difficult, but usually the move pretty quickly (20 minutes or less).
WikiTravel recommends leaving 3-4 hours for transfers from a domestic US flight to an international flight at JFK, which is wise if you think there might be weather or other delays on your first flight.
So here's a breakdown of your time if nothing is excessively delayed:
- Get off your plane at 9am, pick up luggage from first flight (25 minutes)
- Take AirTrain from Terminal 3 to Terminal 4 (includes time to walk from baggage claim to AirTrain, wait a few minutes for the train, time on the train, and walking to the main part of Terminal 4) (25 minutes)
- Check in to international flight, check bags (15 minutes, assuming there isn't a long line)
- Go through security, walk to gate (25 minutes)
For a total of 90 minutes, leaving you 70 minutes to spare. (I rounded your 8:55 arrival time to 9am.) That's plenty of time if your first flight doesn't get delayed.
It ultimately comes down to your comfort level and a cost-benefit analysis. Do you get stressed out if you don't have a ton of cushion time? Is it really expensive to rebook tickets at this point so that your flight into JFK gets you there earlier or your flight out of JFK departs later? Or does the cost of rebooking outweigh your peace of mind with the amount of time?
If it were me, I'd probably keep the itinerary as it stands (unless you're traveling in January when there is a strong likelihood of snow), but it's really up to you and how adventurous you're feeling. :)