According to Sane, your HP 4c is know to work. See this link.
For the Epsom, here is the link --> http://www.sane-project.org/man/sane-snapscan.5.html
This backend is supposed to work.
These are both SCSI scanners, so you will need SCSI support installed, in the kernel or through modules. Since SCSI devices are not so popular any more, it is quite possible some of these distros no longer have SCSI support in their kernels. That is the obvious thing I would look into.
If you decide to buy, make sure you research the Sane's list of supported scanners. Scanner support seems to be very slow coming. I have an old HP 3200C that works ( parallel port ) and a new HP that doesn't work. And probably won't unless I decide to try to write the driver... I checked the list, and could not find much in the stores in my area that was listed as working.
Good luck, getting the old ones to work, or buying a new one.