Except for assessments against responsible persons under section 3734.908 of the Revised Code, no assessment of the fee imposed by sections 3734.90 to 3734.9014 of the Revised Code shall be made by the tax commissioner more than four years after the date on which the return for the period assessed was due or was filed, whichever date is later. This section does not bar an assessment when any of the following occur:
R.C. §3734.909