I think the main reason I still have a hard time characterizing WN as part of the "Big Four" is because they have no alliance and such a small regional footprint.
Of course, learning that they collude just as much as the other three cartel members somewhat reduces my reluctance to admit them to this "club".
It will be interesting to see if one of Frontier, Allegiant, or Spirit ever attempts to clean up its image, offer halfway passable customer service, and attempt to behave like Southwest did many years ago. (It was no-frills, but prided itself on being customer-friendly.) It seems like Frontier might have a little bit of this in their DNA. They *were* this once. I don't expect Allegiant to do it...they're just going to keep duct-taping up old MD80's and flying them to Florida. Spirit talks a growth game, but hasn't shown any indication of ever behaving like a serious, professional airline (even a no-frills one).