Go to any airfare web site (see below) and use the "multi-city" option, then you put: (let say from NYC)

NYC>FRA – April 09
FRA>JNB – April 16
JNB>CDG – April 23
CDG>NYC – April 30

FRA/Frankfurt
JNB/Johannesburg
CDG/Paris
NYC can be EWR/Newark or JFK/Kennedy

This route to-day will cost you:
USD1.991.00 all in

Also try:
http://www.airbrokers.com/html/around_the_world_tickets.php

http://www.airtreks.com/

Please check:
Tips:
a- use flexible dates
b- use alternative airports

http://www.farecompare.com/

http://www.kayak.com/

http://farecast.com/

http://www.airfarewatchdog.com/

http://www.cheapair.com/

http://www.cheapoair.com/

http://www.bookingwiz.com/default.asp?mic=5ruwex6&prod=air&gclid=CIi2xd3HmIoCFSHsPgodDg7Qjg

http://farechase.yahoo.com/

http://airfarehawk.com/

http://www.sidestep.com/

http://www.airgorilla.com/

http://www.vayama.com/

http://www.lessno.com/

http://www.orbitz.com/

http://www.expedia.com/default.asp

http://www.travelocity.com/

http://www.hotwire.com/

http://www.priceline.com/

http://www.travelzoo.com/

http://www.shermanstravel.com/

http://www.placestostay.com/
References :

Sometimes, an airline will offer a fare that allows a stopover on the way with little additional charge. I suggest that you find a good travel agent and explain your situation. A travel agent can research the fares (and there may be a lot of them) and tell you if your plan is feasible and how much it will cost.
References :
Formerly worked in the travel industry