I didn't even know this was a thing until it happened to me randomly one night last week.
I was super confused at first, but found it a load of fun. In all the time I've been playing (daily, since it was released), I've only had two 4v4 matches when I join via Quick Match. I'd like more as it's complete chaos trying to hit the opposition while avoiding your team. It's just so crowded that it's nigh on impossible not to take out a friend.
4v4 would be amazing on the new Conveyor level that dropped today, I reckon.