If there was a fourth piece, it would have been a part of the legend, as it was stated in the creation story in OoT.
If there was a fourth Goddess who created a piece later, it would not have been a part of the triforce, but more likely an element of its own (like Force Gems).
If there was a fourth piece that the Hylians knew about, enough to depict on their Hylian shields, then why is no legend attached?
And if there was a fourth piece, tying together the three outer pieces that represent courage, wisdom and power, what would that fourth piece even represent?
The idea of a fourth segment of the triforce only raises more questions than it answers.