What is the difference between MX RGB and this?
Not sure what you are referring to. In the previous Ducky Shine 3 the spacebar area had customizable color lighting options for that particular switch separate from the rest of the keyboard the Shine 4 does not. This has nothing to do with MX RGB.
Sorry, it's off topic, but I'm struggling to see the difference between Cherry MX RGB and manufacturer implementations of RGB backlighting with MX Switches like this ducky.
Again see photo I edited in above, the shine 3 had 3 separate led's in the spacebar area to make different color options by combining the 3 colors.
The cherry mx RGB clear case switches as implemented by corsair use rgb smd leds on the PCB to enable multiple colors theoretically for each switch depending on software implementation. The point of the cherry mx rgb switches is that they have a clear case and the led is no longer in the switch but on the pcb and can be illuminated and diffused by the entire switch casing.
The ducky shine 4 fire edition has a few different dual color led's but that is also different again.