The free download will be part of Nintendo's celebration of The Legend of Zelda's 25 anniversary.
The upcoming free DSiWare version of The Legend of Zelda: Four Swords is the Game Boy Advance multiplayer game that was included with the release of A Link to the Past on the GBA. We confirmed this with a Nintendo rep during E3.
The DSiWare title will be available for free to gamers in celebration of The Legend of Zelda's 25 year anniversary and will be released in September.