Not sure if you're still looking for an answer, but I got my ScanJet 4600 to do some scanning. I just switched from Windows to a Mac, so I didn't want to buy a new scanner just to scan...
After searching, I found a topic that talked about a work-around to have the drivers to work with Leopard, I hoped that it would work with Snow Leopard as well. By accident, I found that it did.
Simply, download the I downloaded the drivers from the HP site for Tiger. Open the file, and once "SJ4600" is on your desktop as an attached drive, open it. Right Click/Control Click on the ScanJet installer, and select "Show Package Contents" Expand "Contents --> MacOS --> Sub Installers". I double clicked "HP Scan Jet 4600" and "HP Scan Pro". From there, I was able to press the scan button on the scanner itself, and it was able to scan into my Mac. It won't be recognized by Image Capture, and I'm not too sure about other programs, but at least there is some functionality.
Hopefully, you can still squeeze some life out of your HP Scanner...