I'm going to go ahead and say R4s are the most bootlegged these days.
You don't see as many bootleg NDS games as you used to see with the GBA. Oh they certainly exist, the odd tale of a fake game bought from the likes of ebay, but those who partake in the illegitimate trade of games know that more consumers are aware of R4s now.
So what's more profitable, trying to sell the fake games, or fake R4s? Naturally, fake R4s.
With fake R4s you take advantage of consumer ignorance.
As a matter of fact, where I live there's a local stall on Thursday and Friday, maybe the weekend that sells R4s and Razor blades.
They're all the SDHC/R4i/Gold models though, and he way overcharges on them, like £25.
It's pretty damn obvious he buys them online and then sells them for a gratuitous profit.
Why isn't he selling Acekard 2i's, despite how much better they are? Simple enough, because everyone knows what an "R4" is. It's the same as "Q-tips" and "Kleenex", they're just brands of cotton swabs and tissues.