Massively dependant on condition, provenance, usability, manufacturer to name but a few. I have seen a few very scrappy single 4's here in the UK go for around 2-3k but there were not examples I would ever choose to use and were generally all black-powder/needed a lot of work. Compare that with a mint condition, cased single 4 with brass cases and a good manufacturer which is easily achieving prices in excess of 10k, if not considerably more.

Craig.