Well since I don't really believe they'll come back after their "break", they might as well just keep it as is.
But if they do, I definitely think they shouldn't add anyone new. I'm sure people would still eat it up, go crazy over the new guy and all, but I feel like it's not necessary. I'd either have it stay as 4 or let Zayn back in if he wants to return at some point (though I doubt he does), especially if they need a push in sales/news stories later on.
