The name and objective of this event is a reference to The Legend of Zelda: Four Swords side-series, where four Toon Links of the colors used in this event battle to collect the most rupees, the unit of currency of the Zelda universe. The Japanese name is a pun on the title of The Legend of Zelda: Four Swords Adventures, which ends in "+", and the title of Brawl which ends in "X".
This is the only co-op event where both players are the same character.