Landing a McCree ult can be difficult at higher elos especially with all the shields and negating abilities in the game. To land four or more kills, you either need a bit of luck, or some great teamwork. And in this case, it was a bit of both.
As McCree set up his ultimate, the enemy Mei put up a ice wall, negating the ultimate entirely. Or that's what should have happened.
Posted by Redditor Jokqer42, you can see how the perfect storm led to his victory. Right when Mei popped her wall, the enemy Sigma must of mistimed their ultimate, causing the McCree to fly straight up in the air, over the wall. On top of that, his own Sigma used their ultimate at the same time, causing the enemy team to fly up in the air, in perfect view of the McCree. All the cowboy had to do is hit the button and snag four kills.
If the enemy Sigma didn't mess up the ultimate, McCree and Sigma would have completely wasted their ultimates.