The punishment is double the normal maximum sanction for a grade B offence.
A spokesman for the Rugby Football League said the disciplinary panel used its discretion to amend the grading because of an aggravating factor.
Shenton sustained an anterior cruciate knee ligament injury as a result of Boudebza's tackle.
The 29-year-old Tigers skipper will have major surgery in the coming weeks.
The incident occurred in the first half of Sunday's 16-16 draw in the first round of Super League fixtures at the KC Lightstream Stadium.
Boudebza, who pleaded not guilty, was also fined £300.
The suspension leaves Hull KR without a specialised hooker, with first-choice number nine Shaun Lunt sidelined after ankle surgery.
Hull KR hooker John Boudebza has been banned for four matches for the tackle that ended Castleford captain Michael Shenton's season.