Four Sword

The Four Sword is a magical sword that appeared in three games so far (Four Swords, Four Swords Adventures, and The Minish Cap) making it one of the most known weapons/items in recent The Legend of Zelda history.

The Four Sword has a long history behind it, told in The Minish Cap. It was originally the legendary Picori Blade, which was broken by Vaati and reforged by Melari into the White Sword. Link, who needed the sword to defeat Vaati, collected the Four Elements and transformed the White Sword into the Empowered White Sword. Once he collected the final Element, the sword became the Four Sword.

The Four Sword has the ability to split its wielder into four copies, making it possible to move larger stones out of the way, or hit several switches simultaneously. The Four Sword can also be used to "seal" evil powers away in itself, which is what Link did to Vaati, and later Ganon. After Vaati was first sealed in the sword, the wielder no longer had control of the sword's splitting powers, suggesting that Vaati "corrupted" the blade somehow.

There is speculation that the Master Sword and the Four Sword are actually the same blade.

Holy Sword
According to The Minish Cap manga, the Four Sword was the Holy Sword and not called the Picori Blade.

Strength
The Four Sword is just as powerful as four Empowered White Swords. When the Four Sword splits Link into four each Four Sword held by the Four Links is equal to one Empowered White Sword each.