Four-leafed clovers draw most of their power from the mystical connotations from the number four (4) which in numerology, appears constantly as the symbol of balance, unity and completeness.

The number three (3) has also had lucky connotations especially in the Christian era, due to the association with the Trinity. Thus a normally three-leafed plant, lucky in itself and so useful in keeping off supernatural evil, would have its power immensely increased with the addition of another leaf.
Horse Shoe
The Number Thirteen
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