4. The apportionment fraction of a sum certified by the actuary to be equivalent in value to the aggregate of the future payments of annual charges and liabilities referred to in sub-paragraph (c) of paragraph 2 of this schedule shall be paid out of the paying authority's fund to the receiving authority by annual payments of such amount including interest as the actuary shall determine, and over a period determined by the actuary to be the mean residuary period for which such future payments are payable.