HPA tank: Carbon fiber?

well, 4500psi is the real reason you're paying so much more. that's half again as much pressure per square inch that the tank can safely hold. CF/FG tanks are a composite tank that uses various materials to make it extremely light weight while very durable and sturdy at the same time. Normally, FC/CF tanks consist of an aluminum core surrounded by layers of carbon fiber or fiberglass to allow the pressure to be distributed evenly across the entire body of the tank. Those fibers are in-turn kept in place by the layer of epoxy resin (the slick-looking clear or black stuff on the outside of a CF tank) which is extremely durable due to the molecular hardening process that epoxy goes through as it cures. The materials and load sharing characteristics they exhibit are the reason CF tanks are more expensive than aluminum ones. They have a higher tolerance and can take a bit more abuse before they must be destroyed due to the construction type.