setori's command does not specify the resolution to use for the tiffg4 output. The consequence is: Ghostscript will use its default setting for that output, which is 204x196dpi.
In order to increase the resolution to 600dpi, add a -r600 commandline parameter:
gswin32c.exe ^
-o output.tiff ^
-sDEVICE=tiffg4 ^
-r600 ^
input.pdf
Also note that TIFFG4 is the standard fax format and as such it uses black+white/grayscale only, but no colors.
@jeff: Have you ever tried the -dDITHERPPI= parameter with Ghostscript? (Reasonable values for lpi are N/5 to N/20, where N is the resolution in dpi. So for -r600 use try with -dDITHERPPI=30 to dDITHERPPI=120).