ATM machines
 

I have a 4 digit PIN number. Will it work in ATMs in the UK, Germany, Austria, Hungary, Czech, Denmark, Norway, Sweden, Finland? Or do I have to add zeros to it?

ATM's in Europe are 4 digit.

Normally 4 digits, but some ATMs accept longer digits.
Your 4-digit PIN (numbers, not letters) will be fine in all the countries you are visiting.
Many card issuers require holders to contact them in advance if they wish to use their card abroad, for fraud prevention. So check.

A 4-digit PIN is fine.

((I))

PS,

Some folks have reported difficulty with 4-digit PINs starting with Zero

I'm UK based and my old Lloyds debit/cashpoint card PIN began with a 'zero', so that shouldn't be a problem for anyone....