Hello, I have the same problem, I come from Luxembourg and we also have 4 figures. I advise you to put as 5th digit a 0, otherwise contact the customer service, which will certainly correct this bug😉
I am from Belgium and when I enter my postal code, the server does not want to recognize it (it is a four-digit number). So, to be able to register, I randomly put a five-digit number and it worked. Is there still a Mode me to put the right postal code?