Hi,
I think you have a revision C1 model, which can't run v4 firmware:
http://www.dlink.co.uk/cs/Satellite?c=TechSupport_C&childpagename=DLinkEurope-GB%2FDLTechProduct&cid=1197319340092&p=1197318962293&packedargs=QuickLinksParentID%3D1197319340092%26locale%3D1195806691854%26packedargs%3Df9ffeaf4e83ab6d0166b8cc93b23ebbc3d83540f46ceb58f26706558f9479c4ec3df1411a95e2d40c86a4a0e0b114caf9127d5918017382687d64b73399cf16d&pagename=DLinkEurope-GB%2FDLWrapper
You can check your hardware revision like this:
http://www.dlink.co.uk/cs/Satellite?c=TechSupport_C&childpagename=DLinkEurope-GB%2FDLTechProduct&cid=1197319340092&p=1197318962293&packedargs=category%3DTechSupportSearch%26locale%3D1195806691854%26term%3DDGS-1216T&pagename=DLinkEurope-GB%2FDLWrapper
Hope it helps!
Cheers,
Tamas