The complaint was made by citing commissioner Peter Larter from England. The independent judicial officer, Jean-Nöel Couraud of France, heard evidence and submissions from Afoa who pleaded not guilty to foul play.
The player was accompanied by Ulster director of rugby David Humphreys, team manager David Millar and legal representative Donal Spring.
The judicial officer also heard evidence and submissions from ERC disciplinary officer Roger O'Connor.
The judicial officer upheld the citing complaint and found that the offence warranted a red card, and employing the IRB recommended sanctions for Law 10.4(j), he determined that the offence was at the mid-range entry point of six (6) weeks.
Having then considered the mitigating factors, including the player's clean record, good character, age and experience, the judicial officer allowed a reduction of three weeks, before imposing a suspension of four weeks.
Afoa will be free to play on Monday, May 7. Both the player and ERC have the right to appeal the decision.
Law 10.4 (j): Lifting a player from the ground and either dropping or driving that player's head and/or upper body into the ground whilst the player's feet are off the ground.
A statement released by Ulster Rugby read: "Ulster Rugby is disappointed with the outcome. John Afoa himself is very disappointed, particularly in view of his outstanding disciplinary record where he has never previously received a yellow or red card during his professional career.
"John looks forward to returning to play after May 7."