I had the same issue and have made the same decision. No more HP products for me!
The scanner was good but is not for everyone. Sad with HP and Apple sometimes.
Nonaki

Info I've gleaned for the 54XX series scanners, might not work for the 4850 though the list of supported scanners has ranges either side.

http://www.ellert.se/twain-sane/

This site lists several binary packages which should make your scanner work for you, too � after you installed them in the following order:

gettext
libusb
SANE backends (installs all, even though you may only need the HP 54XX one)
SANE Preference Pane
TWAIN SANE Interface
After installing all these, go to the SANE Preference Pane and disable all the backends you don�t need (at least that�s what I did; it works for me, but I don�t know if it�s necessary). Next I clicked on Configure and then OK; without this step, apparently, the SANE/TWAIN interface is not activated. After closing the preference pane and entering my password to store the settings, I opened Image Capture. A preview dialog popped up and my already connected scanner did a quick preview scan. From there I was able to scan the document on the flatbed.

If this does not happen for you, make sure your scanner is properly connected. By going to the Apple Menu > About This Mac > More Info, you get access to the System Profiler. One of the last entries in the left column under hardware should be USB. After selecting this entry, the right panel should show the USB Device Tree. Make sure your scanner shows up in that list.