And that is assuming I belive your assertion that Marines can't hit someone without nearly killing them.
Of course they can and often do. However, with:
1) Training in lethal combat (time taken to disable / destroy opponent before moving to the next is life or death in the field of combat)
2) Fight or flight instinct which is trained to normally default to fight, as well as the huge adrenaline spike when in a situation where yours or your buddy's life / health is in jeopardy.
3) Four such trained individuals backing each other up, as they are trained to do
4) The necessity to make sure that the target is no longer a threat when possible (sure, you broke his arm; but he can use the other quite well if he has a concealed sidarm / another knife, so break his other arm and he's no longer a threat, unless you can successfully pin him so that he is no longer a threat)
etc... all in a matter of seconds