business visa exit and entry

I have a multiple-entry 120-day business M that expires Feb. 21. I know I need to leave China and return before that date in order to extend the visa for an extra 120 days extending past Feb. 21, but I want to know whether during those 120 days (past Feb. 21) I can leave the country and come back, or whether I'm not allowed to reenter after Feb. 21.