Steyn was red-carded in the Sharks 12-11 Super Rugby victory over New Zealand's Waikato Chiefs in Durban on March 21 for a tip-tackle on flyhalf Aaron Cruden.
SANZAR judicial officer Jannie Lubbe found Steyn not guilty in the disciplinary hearing that followed, saying his initial tackle was legal and that the force of two team mates entering the incident led to Cruden making contact with the ground in an awkward fashion.
SANZAR appealed the judgement and announced their Appeals Committee has found the Sharks flyhalf guilty.
"The Appeals Committee unanimously upheld the appeal brought by SANZAR and found that Steyn breached Law 10.4 (j) and that the referee was correct to red card the player," SANZAR said in a statement on Tuesday.
"The Committee determined that the relevant sanction was four matches. After taking into consideration the Sharks' bye in Round 11, Steyn was suspended from all forms of the game up to and including 3 May 2015."
Steyn was one of three players sent-off in the ill-tempered encounter. His team mate Bismark du Plessis was suspended for four weeks for striking out at an opponent with a boot, while Chiefs hooker Hikawera Elliot received a one-week suspension for leading with his shoulder into the head of Sharks prop Tendai Mtawarira.