See for yourself:
http://screenshotcomparison.com/comparison/188207
The Blu-ray is sourced from a higher quality print, but has of course been encoded to less than 2k quality. However, it has been sharpened, which leads to the illusion of greater detail.
The 4K scan shows levels of detail that approach that of the Blu-ray, if not surpass it in some cases. However there are also areas where there appears to be less detail than in the Blu-ray, though this may only be an illusion caused by sharpening.
The color is really what separates these two scans in my opinion. Try as I might, I often can’t recover the subtle greens of the Tantive walls in my project, nor can I recover the yellow hues specific to C-3PO, which is why I may need to use other sources to achieve these gradients. The 4K scan has none of these color issues, at least in this shot.