The Four Sword is a magical blade that has the power to create up to three identical copies of whoever wields it. The sword makes its first appearance in The Legend of Zelda: Four Swords, though chronologically it makes its first appearance in the Minish Cap where you witness the creation of the sword. The Four Sword is forged from the remains of the broken Picori Blade by Melari. It is initially called the White Blade, but later enhanced by the addition of four elements (Earth, Fire, Water and Wind) to become the Four Sword.
In the two Four Swords games the replicas can be differentiated between by their colour (Blue, Red, Purple) though in the Minish Cap the duplicates are just a blue colour. The Four Sword has the ability to shoot blasts of energy at enemies when Link has full health. The Four Sword has the ability to break Vaati barriers. In Four Swords the blade immediately splits Link into four when he uses it, but in Minish Cap the duplicates are only around for a limited time and you can only create copies of yourself in certain spots. This mechanic is used for both puzzle-solving and boss encounters.