There was a bug in several places (ESG included) where a mob death was being multiplied by the number of players present for spawning purposes. It's been fixed. Now you need to kill the same number of mobs whether you're in a full group or solo. It was not intended that you'd have to kill 1/4 as many things just for being in a group. Presumably it's still faster for 4 people to kill them than 1.