I always thought 4c was made up. Most folk who say they have 4c ususally say it because they see someone who claims 4b but the hair has curls like a 4a. So then the 4b person feels that they can't be a 4b if this defined coily person is claiming 4b so they call themselves 4c or 4z to mean that they do not have any curl definition at all.
In reality, some people claim 4b when really they are 4a. Some folk are 4a/4b but claim 4b but they still have a wave/coil pattern. And some 4b's have fine hair which looks different than a coarse haired 4b. And thick 4b looks different than thin 4b. Just as with any other hair texture different folk's hair will look different depending on manipulation, health, products used, thickness and coarseness of the hair.