User Activity

I'd like to have the option to scan photos at 4800 dpi.  Literature I've read say it's possible with this machine, but when I use Scangear, the max I see is 1200, even though I've enabled large scans. In advance, many thanks for any help.  
When I go to scangear, advanced mode, preferences, scan and try to select "enable large image scans" I cannot, because it is greyed out.  What am I missing?