Some foreign ATMs require a 6 digit PIN. Don’t worry, your 4 digit PIN will work just fine at these ATMs.
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 code followed by 00 (or 0) and the system should accept the code. The first 4 digits are the most important and it is what the ATM uses to recognise that the correct PIN is being used.