The most likely issue is that you’re scanning a photo print rather than the original film/negative. A print already has reduced detail and dynamic range compared with the original, and scanning that print can exaggerate surface texture and other artifacts, which often become more visible in dark areas.

The matte finish may also contribute, since the scanner is reading the print’s surface as well as the image. Changing dpi alone usually won’t fix that.

If you still have the negatives, the best solution is to scan those instead, ideally with a scanner designed for film. That will usually give much cleaner results, with better detail and tonal range, than rescanning a 4x6 print.

If the print is your only source, you may need to accept some quality loss and do minor cleanup afterward.