The Legend of Zelda: Four Swords Anniversary Edition, previously downloadable for DSi, DSiXL and 3DS, has both a single player and multiplayer modes. Is there anything unlocked or any benefit to completing single player mode?
No, there is nothing uniquely unlocked by completing the single player mode that is not available by completing the multiplayer mode. The benefit of the single player mode is that unlike in the original release on the GameBoy Advance, this version includes a single player mode and can be played alone. The only other "benefits" to the single player mode aside from the fact that you don't need another person to play is that if you're looking for a challenge, the game can be harder as there's only one of you.