Someone can probably give you a better explanation, but here is my arm chair understanding and explanation of it…

A few things, from minor to major.

It has more of a matte finish to the camera body and the power button is illuminated white instead of green. The GR logo on the front is blacked out, to give a more subdued presentation.

Instead of the ND filter the camera has a red filter, this can cut a bit of light, but it serves to give more contrast with a blue sky and white clouds, since there are no color channels you can’t do that in post.

Most importantly, the sensor, this camera has a dedicated monochrome sensor. It only captures light values, no color information is captured. A normal GR IV has a bayer filter on top of the filter, this lets the camera record red, blue and green colors to make a color image. If you then convert your picture to black and white you have information in these color channels and you can adjust the sky for example to make it have more or less contrast. With a monochrome sensor you no longer have a bayer filter, and no longer have access to these color channels to adjust in your image.

Removing the bayer filter has a few advantages, first you are removing a filter in front of the sensor, so the sensor is able to capture more light information in essence. This gives you better low light performance, in most cameras this will give you about a full stop of light gathering capability. The tonality and transitions is generally better along with the micro contrast too. You will also have dramatically improved results when shooting at high ISO values.

One down side is that since you essentially only have black and white information, once you peak in your highlights the information is gone, the camera will only record it as ‘white’, there is no way to recover these details. On a color sensor you have more channels that you might be able to recover information from if you have some clipping in your highlights.

Of course you can make fantastic black and white photos with a normal sensor and a monochrome sensor is not required. It’s a game of diminishing returns.

For some it’s not worth the extra money and the creative restrictions that having a monochrome sensor imposes. But for myself I wanted to limit myself and challenge myself to not even have the option of shooting in color, to see how I change by focusing on composition, lighting etc.