Lets look at the regulations. In all likelihood, he was charged under
10.4(g) Dangerous charging or knocking down an Opponent with the ball, including shouldering
Regulation 17 Appendix 1 says;
Low End = 2 weeks
Mid Range = 5 weeks
Top End = 10 up to a maximum of 52
Contact was made with the head, with force, so Low End is out of the question. It was not serious enough for Top End, so I reckon it was Mid Range - 5 weeks with a week off for an early guilty plea. There would be no mitigation for a clean record, because he has been yellow carded for this offence before.
Four weeks looks about right to me.