You can concatenate character '3' and the original value then format it in four digits padded with zeros using the StringFormatter, as in:

Thanks, @takashi your answer helps me, but what about if I have the original value already 4 digits ( in this case I don't need to add the "03" at start of the attribute

You can use the StringLengthCalculator to get the number of characters and use the Tester to filter out the feature if the number was 4 (or more).