Ah, so you're dubbing the forth triangle the Triforce of Darkness, eh?
Let's think about this.
There are Goddesses and Triforce pieces of Power, Wisdom, and Courage.
It'd make sense, for a fourth Triforce to relate to a Goddess. But who?
I never heard of a Goddess of Darkness, but in MM, Princess Zelda refers to a Goddess of Time. In PH, Ciela is revealed to be the spirit of both Courage and Time. Hinting at some connection between Farore and whoever this mysterious other goddess may be - if she exists. The Desert Colossus is also referred to by Sheik as the Goddess of Sand.
Ganon is the King of Darkness, so having a Goddess of Darkness may be redundant. But, up to you. Goddess of Shadow? Goddess of Twilight?
Another possibility? The Oracles. Din, Nayru, and Farore. Named after the Golden Goddesses. So,
Din = Seasons = Power
Nayru = Ages = Wisdom
Farore = Secrets = Courage
Zelda then reveals herself to be the Oracle of Hope. A fourth Oracle. So, is there a fourth Goddess? Is she named Zelda?
Zelda = Hope = Time?
Or not. Because there is the spirit Ordona. And that name must relate somehow to the goddess. Eldin, Lanayru, Faron.
In Ocarina of Time, Zelda is also revealed to be a Sage. Given that all the Sages are the "Sage of (something that relates to a temple)", Zelda could be the Sage of Time given that Temple of Time is the only temple without a known Sage.