When wanting to ensure that a registration comes from Queensland (perhaps for funding purposes) you may find the QLD Postcode Validator useful.
Edit your postcode field, ensure the field is of type ‘Integer’ and choose the validator “Queensland Postcode”
Edit the Form or subform to make the field mandatory
Save and publish the form (not subform) to make the changes take effect.
This validator uses actual postcodes from the Australian Post Office, not just the range 4000-4999