I was comparing my new 4D (just received it, the one I won for participating in Virtual Freestyle 15) to my old and faithful 3D, they seem very much alike, almost the same size and very similar geometry and construction, but they definitely don't fly the same, the 4D can do some tricks that were not available to me with the 3D (the backspin, the lateral roll, axel-to-fade), but for now, I wasn't able to do some basic tricks with it, like the pancake, so, no flic-flac, not flat 540s, etc. I guess I just need to play some more with the bridle adjustments and get used to its particular ways.
While comparing the 3D and 4D I decided to try to fly them both together in a stack, so I improvised a very simple string to attach the 3D to the 4D, and surprisingly, it worked on the first try! If you look carefully at the pictures, you'll notice a simple piece of bridle string going from the upper spreader connector to the lower spreader connector on the back of the 4D, the line of the 3D is attached to the middle point of that string, and that's all. Here you can watch a video of this experiment:
Best regards from Argentina,