For the 4 years after the expiration of the 8-year valuation period, the valuation for purposes of computing the assessed valuation shall be as follows:
For the first year, the base year valuation plus 25% of the adjustment in value.
For the second year, the base year valuation plus 50% of the adjustment in value.
For the third year, the base year valuation plus 75% of the adjustment in value.
For the fourth year, the then current fair cash value.
35 ILCS 200/10-50