Sounds like some software problem.
Are you sure it's not running out of memory??
When the manual talks about 4Mb fast mem, it's probably meaning 4Mb of
fast mem FREE, not in total.
You usually need a heap of memory while scanning anyway as the
unprocessed bitmap image from the scanner is often held in fast mem.
You could try other software such as BetaScan or FXscan.
Though both of them need about 10mb of fast mem and OS 3.0 minimum.