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.