The amount of duty they need to collect if you are bringing back 4 liters is so minuscule that they don't want to bother with the time to do all of the paperwork. Over the years I have exceeded my allowance many times, always declaring of course, and have never been sent over to pay duty.
I'm sure it would turn out very differently if you were selected for a random inspection and they found 4 undeclared liters in your bag...