You cannot "convert" MPEG-4 multimedia file (.mp4) to Nero CD Cover (.cdc). However, some .mp4 files can have cover embedded and you can most likely use Nero to edit them and save them in the CDC format. This is probably the only way how mp4 to cdc conversion might work.
Convert MPEG-4 video file format to Nero CD cover.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert mp4 to cdc on Windows
A popular burning and multimedia suite from Nero.
Convert from mp4 file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.