I do not know if you can mod the Windows 8 drivers to work with the 4870 or not. The PC I have my 4850 in is just a Media Center PC so I have never installed Windows 8 on it. I did use the Windows 7 drivers on my 6950 rig at one time and it seemed to work ok the only thing I lost was native 3D as far as I could tell. My best guess would be to give them a try and see if they will work with it. I used them on the Windows 8 release preview at one time to see if they were any better or worse than the native Windows 8 drivers as I had one game that just would not work with Windows 8.
My best guess is that AMD is not working on Windows 8 drivers for anything lower than the 5xxx series at least at this time.
This is unexpected and sucks. I went with win8 over win7 simply because I tried the preview and didn't mind the UI changes. I wasn't thinking about having driver issues with mainstream graphics cards that can still run today's games (I just upgraded from a core 2 duo machine to an i5 build and hoped to keep my 4870 for another year before upgrading that).