The most obvious choice for c4d to obj conversion is to use the original software - CINEMA 4D, which offers users tools to export their graphics to several formats, including OBJ format. The support for CINEMA 4D is somewhat limited, so you won't find many 3rd party alternatives or dedicated c4d to obj converters.
Open C4D file Open OBJ fileA CINEMA-4D converter
A 3D CAD translation software