Markjay, to expand re early NTG4's, I know it's early W/S204's with a build date before 11/2008 (like mine!) have the small nav partition on the Comand HDD so can only be updated to V12 2014-15. However, I believe NTG4 is also fitted to E class models as there are update DVD's with a part number starting A212 rather than A204, and do not know if the partition size problem is on any of those as well. The joy of NTG4 is that the DVD's are re-useable, so it's possible to buy a genuine MB DVD set as "second user", install the update then sell them on.
I believe Comandonline have bench tested a fix for early 204's that allows later updates to be installed, but have not heard of it being tried on a car - or what it would cost. If they were closer to me I would offer my car as a guinea pig for them.