My Bank of America account has a six digit PIN, and it has worked at every ATM (BofA or not) I've encountered.
When dealing with 'foreign' ATMs (whether domestic or foreign), my pin is sometimes truncated -- anywhere from 8 to 4 digits.
I've also come across ATMs where it won't work.
Bank of America advises you to change it to a 4-digit PIN before traveling internationally (and actually have a paper handout they'll give at the teller window saying such if you tell them you're traveling internationally.