820 ILCS 405/2207
No determination and assessment of contributions, interest, or penalties shall be made, and no action for the collection of contributions, interest, or penalties which is not based upon a final determination and assessment shall be brought against any employing unit, more than four years after the last day of the month immediately following the calendar quarter in which the wages, upon which such contributions accrued, were paid. This paragraph shall not apply to any employing unit which, for the purpose of evading the payment of contributions, interest or penalties, has willfully failed to pay any contribution, interest or penalty, or part thereof, or to file any report, when required by the provisions of this Act or the rules and regulations of the Director, or has knowingly made a false statement or knowingly failed to disclose a material fact.
Commencing July 1, 1951, whenever the interest provided for in Section 1401 on contributions in any quarter, has accrued to sixty per cent of the amount of the contributions due from any employing unit for such quarter prior to the payment of any part of such contributions, no action shall be brought, or determination and assessment made, against such employing unit for collection of the interest in excess of said sixty per cent of such contributions; provided, however, that nothing herein contained shall be construed to act as a limitation upon the collection of any interest which has accrued prior to July 1, 1951.
820 ILCS 405/2207