D2 Mathematics - Network Flows

Does anybody know how to deal with an undirected arc with both a minimum AND a maximum capacity?

I have googled it but I can only find university resources, which are papers of 100 pages, which I don't have time to read.

January 2013, the paper has one and the mark scheme lets a flow of 0 go through it when its minimum capacity is 4? It states that it can be treated as two separate arcs in the examiners report, however. So I came to the conclusion that it meant 4 flowing in each direction, hence 0, but how would that satisfy the minimum capacity of 4?

Links to paper, mark scheme and examiners report. Q4 (vi)

http://www.ocr.org.uk/Images/144616-question-paper-unit-4737-01-decision-mathematics-2.pdf

http://www.ocr.org.uk/Images/142377-mark-scheme-unit-4737-decision-mathematics-2-january.pdf

http://www.ocr.org.uk/Images/143145-examiners-reports-june.pdf