It's not at all what we are seeing in the dpr studio test scene, so I'd take this comparison as a non-representative test.
The dpr test shows the gr4 to have sharper centre and sharper corners.
From my perspective, the GR/GRII had the best lens. With each subsequent iteration, Ricoh has simplified the lens construction to save space and thus focus faster.
In the GRIII, a lens group in the front was combined into a single lens, resulting in some loss of sharpness and contrast. Above all, the GRIII has much more chromatic aberration (I never saw color fringing in my GRII, but I often see it in the GRIII).
With the GRIV, the design was made even more compact (for AF speed). To compensate for the optical weaknesses of the GRIII, an additional lens was added at the end (field flattener), similar to the lens construction of modern smartphone cameras. This allowed the presumably now missing microlenses on the sensor to increase the sharpness and shadowing of the outer corners of the image. In the test chart, I also think I can see that the GRIV is now fundamentally sharper than the GRIII, but not sharper than the GR/GRII.
However, it appears that a decision was made in favor of smaller lateral chromatic aberration but to the detriment of more longitudinal chromatic aberration. Noticeable color fringing can be seen in some images in the dpreview image gallery.
In addition, the new antistatic coating on the sensor seems to produce a kind of halation/diffusion filter effect, as Samuel explains on his YouTube channel.