In a case not involving a false or fraudulent return or failure to file a return, if the administrator determines at any time after the tax is due, but not later than four (4) years after the return is filed, that the tax disclosed in any return required to be filed by this chapter is less than the tax disclosed by the administrator’s examination, a deficiency shall be determined; provided, that in a case where the federal generation-skipping transfer tax has been increased upon audit of the federal return, the determination may be made at any time within one year after the federal generation-skipping transfer tax has become final. For purposes of this section, a return filed before the last day prescribed by law for filing the return shall be considered as filed on the last day.