Earlier quoted context omitted.
Maybe not a 4 year old, but NYC kids grow up way more independent than in many places in the US. Middle school kids taking public transit by themselves is pretty normal.
It was rare enough that allowing a kid to do that made Lenore Skenazy [0] famous not that long ago... [0] https://en.m.wikipedia.org/wiki/Lenore_Skenazy
If you go back to the 1980s, what she did wouldn't have been that remarkable. In that time, kids like me were gone from home for hours, walking or riding bikes for miles away from home, and we didn't have cellphones for our parents to keep tabs on us. Statistics show that crime rates were much higher (they peaked in the mid-70s, and have been falling since); the difference is that Americans just weren't paranoid back then the way they are now.