Try some B&W. Color film has multiple layers which makes it inherently less sharp than B&W. I don't know what lens, camera and film you used but at 4000 spi I suspect you are at or beyond the resolution of the image.
Sandy King has posted elsewhere on this forum that with the Mamiya 7, a sharp B&W film, and careful technique you would have to scan at about 5400 spi to extract all the available information on the film. As far as I am concerned, this is the limit, at least for resolution (I'm disregarding TechniPan here, which is no longer available).
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