The committee found that the act warranted a suspension of eight weeks, but cut that in half due to his guilty plea and the player's good judicial record.
Van der Merwe was involved in a fist fight with rival Bulls hooker Schalk Brits, for which both players were red-carded in the Pretoria-based side's 19-16 victory on Saturday.
He pleaded guilty to the offence, which saw the usual six-week sanction cut in half.
Brits has also been charge by the Foul Play Review Committee, but no verdict has yet been reached in his case.
Van der Merwe and Brits are likely rivals for one spot in the South Africa squad for the World Cup in Japan later in the year, with Malcolm Marx and Bongi Mbonambi ahead of them in the pecking order.
(Reporting By Nick Said; Editing by Christian Radnedge)
