FYI, the 4" rule is an FDA regulation, and it's illegal to sale turtles under 4" unless for "bona fide scientific, educational, or exhibitional purposes, other than use as pets." Most shows have them because enfourcement is lax and the FDA and local officials have better things to do.
NYC itself has a health code that prevents the sale of turtles under 4". Again, enfourcement is lax, but if more people made a stink to the city council and mayor's office about it, something might be done.
Some areas have stricter rules than that. In MD it's a health code reg that prevents ownership of a turtle unfer 4", although we might have new regs in place this year that will allow breeding with a license. In one county in MD it's illegal to sell turtles, period. In Indiana, it's illegal to sell RES, because they are a native turtle, and so on.
Katrina