Diablo III's latest patch is now live, and among other things, it introduced four new difficulty levels to the game. While Torment VI wasn't that tough, the new hardest setting, Torment X, when played properly, makes the game look like a bullet hell shmup instead of a hack and slash RPG.
Here's a two-minute example, probably the best one out there right now, from Desolacer and his friends. They managed to complete a Nephalem Rift in under two minutes on Torment X difficulty and on Hardcore (which means they don't respawn when they die). An impressive feat only after a few days of Season 4.
Easy to understand now, after watching this (or any similar videos with players doing high level Greater Rifts on Torment VI), why Blizzard limited the game to four players, even on PC. This would be crazy to follow with 8 or 16 players on the same map.