You cannot exactly convert .c52 to .c04 files. Both are just one parts of a split multi-volume WinACE archives. You need to gather all the pieces and decompress them with WinACE or any other archiver that supports this format. There really is no other way to do it.
Convert Split Multi-volume ACE compressed file archive to Split Multi-volume ACE compressed file archive.
We found one software available in the database for this conversion.
Microsoft Windows software - convert c52 to c04 on Windows
A compression utility for Microsoft Windows
Convert from c52 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.