I think it is the other way around. Some us banks use 5 digits, but the overseas ATMs are 4.

In Seoul I have not had major problems, although not all banks have Global amts. Many that are global will say global on them, but some do not. Sometimes there will be a row of 4 machines and only one will be global. Usually you can tell that one is different. If traveling outside of a major city, get cash before you go. It can be difficult to find an ATM and the acceptance of CC may be limited.

IME, Japan has the same issues. I had a heck of a time finding an ATM at the Big Egg that would take my card. Fortunately there are seemingly dozens and finally I found one.