To pull this off, you will need a full bar for your focus shot. The problem is that there are not that many areas of the game were you will be facing off against four or more animals aside from the bird swarms while you are climbing cliffs. Birds are not easy to hit, but the birds present the most opportunities to get this done. When you have a full bar, wait for a flock of birds to come at you, pull the left trigger ()and down four birds. You'll have numerous opportunities to do this through out the game.
If you don't want to mess with trying this on birds, there is a good spot to get it right after finding Dr. West and getting the "Here You Are!" achievement. After finding her you're tasked with hunting for the sake of getting fur to keep Dr. West warm. Right after this segment starts you will walk a short distance and get ambushed, this ambush has 4-5 enemies and if you're using the bolt action rifle you can kill them all in one shot. If you fail just restart checkpoint. There are also 2 more similar sized ambushes to attempt this on right after the initial ambush.