In my opinion and according to statistics involving injuries and deaths four wheeler ATV's have caused more injuries and death than dirt bikes.
Also a four wheeler or ATV has the potential to topple onto someone which can pin them beneath the ATV which is one reason they cause death more than dirt bikes do.
Both four wheeler and dirt bikes can be safe as long as you use them correctly and wear the proper safety gear including your helmet.
Practice safe riding and if you have kids teach them safe riding techniques as well and they're pretty safe.