Hi Johnman,
As far as I can see, the Ref Code is merely that, a reference. It is not intended that the specs limit or regulate the operation of an A/C, but act merely as a guidance to airport planners when planning/building an airport.
I would also imagine that a code of 4C is unlikely. I am not sure, but I think that you would see either 3C or 4D/E/F.
I think that the code numbers and letters go hand in hand. i.e.: 1A, 2B, 3C and 4D/E/F.
Check out
www.caa.co.za then go to "manuals & procedures" then to "Navigational Aids" (acrobat reader) and then to page 2/22. Here's a table with some info for you.