The Four Sword is a weapon encountered by Link in four games. It allows the wielders to split into four copies of themselves. Vaati was sealed inside the sword before escaping twice. In The Minish Cap, Link must forge the Four Sword with the Broken Picori Blade and the Four Elements, infusing them at the Elemental Sanctuary.
The weapon was originally forged by the Minish as the Picori Blade and given to the Hero of Men to fight off monsters that overran Hyrule. The Sword was used to seal the Bound Chest containing the monsters that plagued the land. It was never touched except every 100 years during the Picori Festival by the victor of a tournament held during the occasion. During the events of The Minish Cap, Vaati was the victor of the tournament, and used this opportunity to break the Picori Blade and unseal the chest, releasing various monsters.
During The Minish Cap, Link ventures off to repair the Picori Blade and infuse it with the power of the Four Elements by using the powers of the Elemental Sanctuary hidden inside Hyrule Castle. Each time a new element is infused into the blade, it gains the power to split the wielder into another copy, eventually reaching its maximum power at Four, transforming it into the Four Sword. (hence its name)
After defeating Vaati in Dark Hyrule Castle, Vaati is sealed within the blade until the events of Four Swords, where a different Link and Princess Zelda accidentally release Vaati from his seal within the Four Swords Sanctuary. Between The Minish Cap and Four Swords, the blade has been upgraded to create four permanent copies of the wielder, rather than temporary.
During Four Swords Adventures, the blade once again is the power behind Vaati's seal, though this time the blade is used to seal both Vaati and Ganon, who is behind Vaati's plot, inside the blade using the power of the Seven Maidens. This is the last time the blade is seen and it is assumed that using the power of the Seven Maidens, the seal is much stronger and permanent.
The Blade is infused with a similar power to the Master Sword to fight off great evil, most notably Vaati. In The Minish Cap, the blade can create separate temporary copies of the wielder to solve various puzzles (such as push objects that are otherwise too heavy for the wielder to push, or press down multiple pressure switches) However, by the time of Four Swords, the blade gains the power to split the wielder for the length of time the wielder possesses the weapon, and the copies have differences than the original, such as different colored clothing (Most notably Red, Blue and Purple) and different voice pitches. Once the blade is fully charged (In The Minish Cap, this requires the ability of two different Tiger Scrolls and Link needs to be at either full health or minimal health, though in Four Swords and Four Swords Adventures, this requires a certain number of Force Gems) the Sword can shoot a beam to harm enemies, similar to the Master Sword in certain games. (such as A Link to the Past) It seems this ability is shared with the Picori Blade, as it can be achieved in The Minish Cap before infusing the blade with all four Elements.