In an interview with Game Informer, Legend of Zelda series director Eiji Aonuma revealed that reports of a new Four Swords for the Nintendo DS are strictly fictitious. A statement from Aonuma in an interview at the tail-end of last year was misinterpreted, thus spawning reports of the fictional Four Swords DS. In the quote in question, Aonuma was in actuality explaining that since his work in Four Swords left him familiar with creating a game using multiple screens, he could easily adapt to a system like the DS and do different things with it.
Other than the recent revelation that The Legend of Zelda DS will not be a Four Swords title, nothing else is known about this game. When new details come about, RPGamer will keep you updated on the latest.