There are discussions of this in CPU collector forums often. If the price is high enough, it's well worth it (to thieves). China appears to be a significant source for counterfeited chips, mainly "Engineering Samples," "Prototype" and "Confidential" supplementary-marked chips. And I can imagine that somewhere along the line, there were a whole bunch of ICs with exactly the same external appearance as the 4004 that could be remarked. If the buyer doesn't functional check the chip or open it up for die inspection, who'd know other than the thief who sold it?