The "c" stands for "consumer" version. The E4 is intended for the professional market, so you will probably need to go to a professional audio dealer to purchase it, and probably you will need to order some of the accessories you get included with the consumer version separately, particularly the custom ear molds that the E4's fit into through your audiologist. Otherwise, exactly the same components are used in the E4 and E4c, so there should be no audible difference between the two.