Strange. When you google “little green boxes” and wordorigins you get this thread, which also refers to a “little green boxes” thread… but where is it? I too am curious about when the fourth box will appear.
I think I created that thread in a former persona. When Dave erased that persona, it may have erased threads that that persona created. Anyway, Dave knows the answer. And, as I remember, he can set it to happen whenever he cares to.