Four Swords was published by Nintendo and developed by Grezzo, and Four Swords Adventures was developed and published by Nintendo. Not to mention that both games are factored into the official Zelda timeline, the answer becomes pretty clear.
If you're talking about the Four Swords games as being similar to other Zelda games, I think they're quite the derision from the classic formula. But what I consider to be the untouchable core elements of a Zelda game, namely the gameplay, main characters and control scheme aren't tampered around much in either game. While both games are multiplayer (an almost alien term to apply to a Zelda title) and feature a Link that can divide into four, they are unmistakably Zelda. What else would you call them?