I can only assume that there were only four due to those Four Elements being the only ones capable of forming the Tornado of Creation. I believe it has something to do with those four elements being prominent in the creation myth, mayhaps.
I can only assume that there were only four due to those Four Elements being the only ones capable of forming the Tornado of Creation. I believe it has something to do with those four elements being prominent in the creation myth, mayhaps.