There's no way there's a fourth goddess/Triforce piece. It was just a design, nothing more.
Why would the games tell us over and over and over again that there's only three if there was four?
If there was four, they would've said so by now, or at least given a reasonable hint, other than a tiny, insignificant symbol on one shield in one game.

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Bomber's Notebook | Secrets of the Minish Cap Library | Theorizing Resources | Aralith's OoT Retelling
Voted as Best Zelda Theorist in the Summer '08 Awards. Thank you to all who voted, and to all who inspired me along the way!