The 32-year-old fast bowler reached his landmark when South African skipper Hashim Amla took a chest-high catch at first slip in Steyn's third over of the match.
Shaun Pollock is the only other South African to pass the 400-mark, taking 421 wickets before retiring in 2008.
Steyn, who is playing his 80th Test, became the 13th cricketer to take 400 or more wickets in Test matches.
He is one of only three currently active cricketers to achieve the feat, joining India spinner Harbhajan Singh and England paceman James Anderson.
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