Literally all you have to do is order a card from amazon. Open up the computer. Stick the card in the PCI slot. Close up the computer. Install the driver off the disc (or just let Windows auto load it). It should be plug and play and take no more than 20 minutes. Google/Youtube should be able to provide you with any instructions if you've never installed computer hardware. It really is super easy.
It's my understand that they are still running Win 98
Its my understanding I can't run anything newer than 98 for the 4b to work.
, so the disc would have to have Win 98 drivers or they would have to go hunting around for those drivers on the web or find equivalent. Or maybe their old Win 98 install disc for generic drivers that may work.
Win 98, based on my memory of it on the laptop and running it in the VM now, either searched for it on the install disc or you could locate it on the HD for it, but it didn't auto load from the web on it's own.
I think it would be far easier to get the card working on a newer rig with a newer OS, run Win 98 in a VM, if indeed they have to have Win 98 to run the 4B and just pass that serial port into the Win 98 vm and run it that way.
Edit to add:
Is the Gerber software from that era 16 bit or 32 bit? Win 98 was a hybrid. May be able to bypass the old software and still use it on a modern system directly. Serial ports do have more leeway in that regard. May or may not work. Just spitballing here.