The Wikipedia has a very detailed and (IMO) well written section on the history of Oz. It seems that there are a lot of inconsistencies provided by Baum himself. In general however, the answer to your question is no, there were not always 4 witches. From The Tin Woodman of Oz:
Oz was not always a fairyland, I am told. Once it was much like other lands, except it was shut in by a dreadful desert of sandy wastes that lay all around it, thus preventing its people from all contact with the rest of the world. Seeing this isolation, the fairy band of Queen Lurline, passing over Oz while on a journey, enchanted the country and so made it a Fairyland. And Queen Lurline left one of her fairies to rule this enchanted Land of Oz, and then passed on and forgot all about it.
The Wikipedia goes on to state:
Thenceforward, no one in Oz would ever age, get sick, or die. After becoming a fairyland, Oz harbored many Witches, Magicians and Sorcerers until the time when Ozma made magic illegal without a permit.
Though it also shows how "no one would ever age, get sick, or die" is inconsistent with what we see in Oz.