The file extension C4D is used by the 3D software Cinema 4D. Maxon Cinema 4D is a 3D construction, animation and rendering program for professional use. The C4D format is the native format of Cinema 4D and includes data from 3D models, used textures and rendering settings.
C4D files are not downwardly compatible and can not be loaded in an older version. The opposite way is usually possible, although partially with restrictions. In addition, it has to be considered that C4D files saved with plugins require this plugin (usually the version number has to be the same) in order to be properly loaded. This is especially important if you want to exchange cross-platform data.
The C4D format is a proprietary format and can almost exclusively be read by Cinema 4D.