A fourth member doesn't necessarily have to be Maya. Although personally I think adding a fourth member would be a mistake at this point, the Kalagirls have been together for ten years now and developed a very close relationship between them, so an extra vocalist would just be weird.
Mai HiME = Hanae
Mai Otome = Yuriko (1 by teh Kasahara)
Tsubasa Chronicles = Muh Eri
Hack SIGN = Emily (2 by Yuriko)
Aquarian Age = Kaori Nishina