If the archive is encrypted, the password is often required at the beginning of the extraction from the first part.
Use the 7-Zip official tool to "Test" the archive. If .004 is corrupted, the extraction will fail even if you have the other parts.
When 7-Zip creates a split archive, it appends a three-digit suffix to the file extension. To extract the data, you have all parts in the same folder.
While "C54" is often a generic user-defined label, it frequently appears in specific technical databases: