Are you sure you're not over reacting a bit?
Obviously it's the wrong cab and not suitable for what you want, and needs to be replaced or refunded with them paying carriage. But: As I read your first post it seemed to me that the chap was actually trying to be helpful. He didn't know how much you knew about these things, and in other circumstances, ie someone else, with a different head, they might have been quite happy to keep the 4 ohm cab.