Some foreign ATMs require a 6 digit PIN. Don't worry, if the ATM is asking for a 6 digit PIN, your 4 digit PIN will work just fine!
You can insert your 4 digit PIN as it is first. In case the ATM doesn't allow you to proceed with only 4 digits, enter your 4 digit PIN followed by 00 and the system should accept it.
The first 4 digits are the most important, these are what the ATM uses to recognize that the correct PIN has been entered.