UPDATE: New mortality numbers for 2014 were released (the government is slow in everything it does). Here are the updated statistics:
Average from 2009 to 2014:
Mass murders (4+ murders) account for 0.2% of all murders, and 0.001% of all deaths. All this talk about how horrible the problem is and how America is plagued with mass murders ignores the fact that we're talking about about less than 30 deaths a year. More people die from falling out of trees.