Dimmukane, the trick is that the puzzle is movable within the frame. Move the puzzle within the frame so that the platform lines up with the one on the left so that the enemy walks onto it, then move it over after it walks onto it so that it falls onto the lower level and walks over to the key so you can grab it. Then move it over to the right so you can make the jump between the two higher platforms and get the 4th piece.
That puzzle, as soon as I figured it out, was what sold me on the game. Beyond the beautiful art, there's just a supreme creativity and attention to detail to the way these levels are designed. The problem is that the game gets fucking hard after a little while. It took me a second to figure out the trick to 2-2, but I spend 15-20 minutes trying to figure out the last piece to World 3 before I just moved on even though the one missing piece really chafed the completionist within me. But I only managed to get about half the pieces in World 4, it's really tough. I turned it off at that point yesterday because I was sure that World 5 would make me feel like an idiot.
Don't get me wrong, I love the game. Yes, it's somewhat pretentious, but it's got a great aesthetic design and the care given to the unconventional gameplay is beyond almost anything I've seen before. It's more expensive than most XBLA games, sure, but a lot of people on this forum bought the Penny Arcade game for even more money and while that was great and all, it wasn't nearly as thought-provoking or unique as Braid is.