I think Five would be the minimum for JL. But im still wondering if they have the balls to just give people Bats, Supes, Wondy and Flash, with cameos by MM and GL.
I don't think they will, but I wonder about Green Lantern these days. Im sure we'll get a new Hal or hopefully John Stewart. But would they go with 4? Just to get GL off their backs and to do something different from Avengers? It's an interesting thought.
Maybe it would work if the first movie were those 4 names, hints at Hal & J'onn and perhaps Atlantis. With Brainiac as the villain or the Legion of Doom. Then Darkseid in the sequel with all 7 of the original members.
I'm almost positive right now that if Chad Boseman doesn't take the Black Panther role, WB will jump on him for Green Lantern. That's what I think will happen realistically.