I like the idea of the fourth piece being Chaos, because that could be embodied by Dark Link. I had an idea a while ago that it could be Leadership, since it could be a combination of the three. It could represent a god or goddess higher than the three goddesses. I believe either Impa or the king of Hyrule would embody the Triforce of Leadership.
I beleive the 4th Triforce would be Justice.
Everyone has their own idea of what justice is. It could be good or evil depending on who holds this triforce.