While that may be a valid point, isn't she in the opening right off the bat? It's kinda hard not to be able to guess which two other girls not in the first episode would be in the main four.
Tenshikuro said: True but who actually pays attention to an opening?
Most people who watch anime.
I have yet to watch an opening on any anime.
And I can guarantee you are one of the only anime watchers in the world this is true of.
While I am about as anti-spoiler as anyone can get, this is well beyond what is reasonable. There was never any doubt as to who the four girls would be, nor was there at any point meant for there to be doubt. This is not a spoiler.