The C4 unless it has had the Geiss modification has a fixed 50mm lens. The C44 has interchangeable 35,50 & 100mm lenses. The Geiss C4 has 35,45,50,100 & 135mm lenses. Just be aware that several of these lenses are rather rare and can command premium prices.
As a side note: either camera should have the shutter cleaned before using. The old grease and oil will make the shutter leaves sticky and cause the link that drives them open and closed to break, and the only source for a replacement is another camera. Rick Olsen's site has instructions for disassembling the cameras for service.