In order to kill 4 enemies in less than 1 second using the crossbow you will need two things:
Bend Time (Level 2)
Crossbow Reload Upgrade
If you do not have both of these, you will not be able to kill the targets fast enough without having them align perfectly.
There are several locations later in the game with 4 or more enemies grouped up, such as Mission 6 and 8, but one of the earliest and easiest spots that you can quickly access from the mission select is Mission 4: The Royal Physician. At the very start of the mission are four guards; 2 talking to one another, a captain near the ledge, and one inside the metal structure. You can easily bend time and kill them all without a problem.
However, if you are worried about messing this up, just knock them out with Sleeping Darts or Chokes then pick each one up and place them next to one another. Once they are piled up simply use bend time and kill them quickly. This method of rendering them unconscious and piling them up can be performed in any other area with 4 or more enemies as well. So if you do not have Bend Time level 2 by mission 4, I suggest doing it in Mission 6 (the mansion) or 8 (the main courtyard). This also works for the Tempest trophy as well and can be unlocked simultaneously.
To see a video of the method being performed, please view the video below: