Oosthuizen shot a 6-under-par-66 Sunday to win the event at Durban. He came from five strokes behind in the final round.
The No. 4 ranking is a career-best for the South African player, who won the British Open in 2010.
U.S. rookie Russell Henley's record-setting win at the Sony Open in Hawaii landed him at No. 50 in the world rankings. Henley finished the event at 24-under-par 256, the best 72-hole score ever turned in by a PGA first-year competitor.
Rory McIlroy of Northern Ireland is again ranked No. 1 in the world.
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