Overwatch 2

When you activate McCree's ultimate, Deadeye, time will seem to slow down for you, and you'll see markers around each enemy in your line of sight. If you wait, the markers will eventually turn into skulls. At this point, you can push the button again and release your attack, which is a guaranteed kill on any skulled enemies. It also can do some damage to anyone else in the area. The best way to get 4 kills is to sneak up behind a group of enemies who are close together so that they are less likely to kill you as you're using the ultimate. This works well on payload maps especially because most people will be looking forward while pushing the cart. It's also good to get on control maps when the enemy team controls the objective since they will tend to clump up on the capture point.

McCree's Deadeye ultimate ability allows him to lock on to on-screen targets, shooting and killing all of them.

During High Noon, McCree will begin to lock-on to targets. Higher health targets take longer to lock on to, as High Noon builds at 200 health/second. You can only lock-on to targets that are in your line of sight, and a red skull signifies that pulling the trigger results in a killshot.

Build your ultimate and then try to flank the attacking enemy team. If you're able to get behind them and use the ability with enough enemies lock-on for the killshot, just pull the trigger and unlock the achievement. It is also advised to be on higher ground to look down on the battlefield in an open area to see more enemies at a time. Keep in mind that D.Va in her mech suit does not count as a kill.

If you'd like to learn more about McCree and this achievement, you can view this video:

As with my other Multi-Kills so far, I was able to get this on King's Row (on Defense). A nice thing about objective gametypes is that in most cases, it will tell you how many enemies are on the objective. So what I did was hide in the archway that overlooks the capture point (the one that the cart drives under). To get in there, you have to go into the leftmost building (from defense perspective) after spawning, and go up a couple flights of stairs. I waited until there were 4 enemies in the objective, then activated Deadeye and peaked out the window. Then you just wait until you have 4 red skulls on people. If someone goes out of sight, the circle turns gray, but stays the same size. This means that you won't need to wait for your aim to line back up if they come back into view.

This should work pretty well on any map with some verticallity, as it gives you a good view of the point, If your team is fighting them on the ground, they're less likely to notice you up high (or run away when they hear you call out). A Zarya or Mei Ultimate can also help, but this also means your teammates are more likely to kill them (if they aren't helping you). The Hybrid maps (maps where you capture a point then escort a Payload) seem to be best for verticallity. Hanamura offense should also work well, as the ending is a very open room.

Don't have a video for this one sadly, but I can make one for the location later if it isn't clear.

How I did it was at the Numbani Payload Capture Zone. If your team isn't very effective at eliminating enemies, there may be a lot gathered around the payload. This is where it works best. It also helps prevent them from capturing the point, if it's not already. Here is the clip of me doing it in a match.