(4)On failure to recover any sum due on account of such tax from the person primarily liable, these may be recovered from the occupier of any part of the buildings or lands in respect of which the tax is due such portion of the sum due as bears to the whole amount due the same ratio which the rent annually payable by such occupier bears to the aggregate amount of rent so payable in respect of the whole of the said buildings or lands, or to the aggregate amount of the letting value thereof, if any, stated in the authenticated assessment list.
