The Elite-C v4 MCU was designed as an alternative to the Pro Micro MCUs that are more commonly used in custom keyboards. The Elite-C features a DFU bootloader and USB Type-C connector.
Elite-C v4 controllers are 1.6mm thick. The B-Stock controllers are 2.0mm thick. There is no difference in functionality between the two types of Elite-C variants. The only difference is the thickness of the PCB.
When flashing the Elite-C controllers, don't forget to use 'dfu' instead of 'avrdude