 As you learn E4, given suitable background information, you acquire justification for P4; moreover, to the extent to which you know that not being in Scotland is sufficient for not being in Edinburgh, you also acquire via transmission justification for Q4. This additional justification is transmitted irrespective of the fact that you already have justification for Q4, acquired by spotting the train station sign ‘Newcastle upon Tyne’. If (iii) were read as requiring that the whole of the justification available for a proposition q were had in virtue of the satisfaction of (i) and (ii), cases like these would become invisible.