RE: Tuff Flight's 4D
The SPAD 3D is a novel and simple design. It is good as a torque roll trainer, for harrier practice and does excellent flat spins. However its lack of side area means it will not knife edge or do good rollers. You've probably found that when you stand a SPAD 3D perfectly vertical on its tail, it will spin up like a top due to the very short span. It's wild!
The 4D excels at very tight waterfalls and knife edge spins as well as learning to torque roll and doing roller practice close to the ground where crashing is expected.
Spads are cheap and fast to build, but they're really not designed to crash like the 4D. The plastic will eventually tear, and aluminum will get bent and need to be replaced long before a 4D will show signs of wear from the same crashes. The short span on the spad also means normal turns aren't as tight and tend to "mush" more than a conventional design.
The 4D has some flexibility in the design that allows for crashes to not break the radio, controls or anything that would requre re-gluing. Consequently, it suffers from some lack of precision at high speed and low speed high alfa knife edge.
The new Tuf-Yak excells in all areas, but does not have quite the durability of the 4D which is meant to be crashed as often as necessary.