I don't have a crystal unicorn either. I usually base what elements I use on whether there is a limited edition animal I am going for. For example, right now I am trying for the Knitten Kitten, which is dark and earth. So I make sure the 4 elements I use include those. That way I can try for it and still have the possibility for a crystal unicorn.
With that said, I don't think it matters at all which four elements you use or whether you change them up.