Today Nintendo has announced that for the next four days, 3DS owners can pick up a free copy of The Legend of Zelda: Four Swords Anniversary Edition in the eShop! Released in celebration of the critical acclaim for The Legend of Zelda: A Link Between Worlds, you have until February 2, 2014 to redeem. Nintendo also promises that "the celebrations will continue with the release of The Legend of Zelda: A Link to the Past, available for purchase on Nintendo eShop for Wii U, and a special Art Academy SketchPad event in Miiverse!".
This is not the first time Four Swords has been available for free on the 3DS. The first time, a few months after the 3DS' initial release, it was available for about five months. This time though you only have a few days, so get on it now!