When you are the spy, you can throw coins to distract nearby enemies in the direction you throw it. For this trick to work properly, you need to be real close to the target of interest. On level 4 where you free your allies, you come to a point where there are two guards facing each other. You need to stand real close to one of them, toss the coin to divert the guard's attention away from the other guard, then silently kill the guard that is not distracted. You should have enough time to kill the distracted guard without raising the alarm.