If you scan film mounted in the film holder, you must use the option "film holder" and not "film area guide", the latter mode switches to the low resolution lens, which focuses on the glass plate.
My original, very crappy scans, were in the film holder and I used film holder scanning mode. When I scanned between two glass plates, I used film area guide, and that looks quite a bit sharper. I don't think I could select film holder in this mode, since the scanner software won't find the holder
I'll try the 4800 DPI. I tried, with medium format, 6400 vs 3200, and found better results scanning at 6400 and downsizing (bicubic sharper) to 3200 vs. scanning directly at 3200. I never tried 4800, so I'll give that a whirl!