I thought there was supposed to be a way to share families, etc, with some sort of in-game function, but I never got it to work. But I don't know if that was exactly... "co-op". Sims 4 might have something like that, but I'd have to look into it more.
Maybe you are thinking of The Sims Online where every person controlled a different character.
But that would be a cool idea for Sims 4 though, just make online "matches" where each player controls a few family members.