In only his third match back after a lengthy groin injury, O'Connell was sent off by French referee Christophe Berdos in the 69th minute of Munster's 22-16 Heineken Cup Pool 3 triumph over their Welsh rivals.
Thomas received six stitches in a cut in his mouth after being pole-axed by O'Connell's flailing arm. The two teams meet again at the Liberty Stadium on Saturday.
The four week suspension means O'Connell will not only miss this weekend's vital return match in Europe, but will also be kicking his heels over Christmas as the Magners League leaders tackle Connacht, Ulster and Glasgow Warriors.
He will be free to play from 9 January, 2011, which will leave him with three weekends of action before the 2011 RBS 6 Nations gets underway.
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