here is an easy way i found to farm the kills just, at the beginning of the game, there is some useful tips to help you out with this.
1.- make a save in the same spot i did,
2.- use the sand to light out the torches.
3.- press
to walk silently
4.- always watch for the guard on your right, (if he see you it's bye bye)
5.- when you are close to a wall always press
to take cover this will make you pass unseen for the guards.
6.- reload the save and repeat.
note 1 : try to avoid the guard is resting in front of you, he is hard to kill with this method
if you follow this steps you will get the achievement in about 10 to 15 mins, for killing the four guards.
Note 2 : for the silent kill you must leave the
pressed wen the guards pass through you.