In Joyce D. Wilkerson's "Clownfishes" she discusses having more then one pair in a tank. Assuming they are all juveniles when they are put in, one will become dominate and one will become her mate the others in the tank will remain juveniles and get along with the resident pair. When you have trouble, is when there are two dominate females in one tank, then you will have to separate them or they will constantly fight. Do NOT buy wild clownfish. Buy only aquarium raise clownfish. Either buy all the same size, small to med, or buy one large and three small; that's your best shot at not having two females in the tank. As stated in a previous post what you want is all the same species. Just because one pair is black and white does not bother the clownfish. They are all Ocellaris and they will readily interbreed and have fertile off spring. 40 gals should be plenty large to keep four clownfish if you only have one female.