From Zelda Legends Wiki
A mythical blade that was used by a hero long ago to defeat Vaati the wind mage. It is able to split its user into four copies. The four shards of this blade, each a different color of the Four Sword, can be found in the Palace of the Four Sword in ALttP GBA.
Origin of the Sword
Legends say that long, long ago, evil creatures appeared in the world of humans. These beasts were powerful and frightening, and they caused trouble for the humans.
The Picori came from the sky, bringing powerful magic to humans: the magic light force and the Picori Blade. With these, the beasts were sealed inside a chest, with the Picori Blade the lock to keep it from opening.
The Picori Blade is also known as the sacred Four Sword. Infused with the elements of fire, water, wind, and earth, the Four Sword is able to separate its holder into four.
In the elemental sanctuary is a pedistal for the Four Sword, and four columns to hold the four elements. When all these items are in place, the elements fill the blade with power.
Touching the Four Sword after it's been at rest in the elemental sanctuary causes a person to "shatter to pieces", becoming four versions of their self. Only by fulfilling the sword's destiny (such as sealing Vaati back into the blade) may the sword's wielder become one again.
The Four Sword has various abilities. People turned to stone by Vaati may be restored by the power of the Four Sword.
Once Vaati's body is destroyed, his dark form appears, and the Four Sword is able to defeat him. Also, the Four Sword is able to seal Vaati away within its forged steel. Similarly, with the power of Zelda and six other special maidens, Ganon may be sealed within the blade.
When the Four Sword loses its power, it becomes the White Sword. When re-infused with elements, the White Sword begins to regain its ability to split its user into multiple selves.