4. Notwithstanding sections 2 and 3, receipts generated from users during a total or partial interruption of the public transit network of any of the public transit authorities of that region as a result of a case of superior force or of a labour dispute having the effect of reducing the receipts of that authority, or of any of the public transit authorities of the region, by at least 4% of what they would have been if that interruption had not occurred may not be considered in establishing the apportionment among the public transit authorities of the same region. That percentage difference is obtained by comparing the receipts generated during the period during which there was an interruption of the network with those generated during the equivalent period of the preceding fiscal year.