So much of the amount apportioned to a county under Section 6-901 that is obligated under Sections 6-902 through 6-904 and for which local funds have been committed under Section 6-905, within 4 years from the date the apportionment is made, shall, upon certification by the Department, be paid to the county treasurer, who shall apply those funds to the payment of such obligations. Any funds allocated to a county under Section 6-901 that are not obligated within 48 months under Sections 6-902 through 6-904 shall revert to the Road Fund.