> Production of the R700 family is winding down. Many models of the HD4XXX series
> are hard to find.

R700/HD4xxx is just one generation behind the most recent ATI/AMD GPU family. Where I live all HD4xxx series cards are still everywhere to buy.

> It would be nice to have a more up to date GPU on a PPC based system

That would only be R800/Evergreen/HD5xxx series.

> but how are you going to get the documentation to write the drivers? ATI has never
> completely documented their GPUs.

I don't know how Hyperion are planning to accomplish this. Work on supporting R700/HD4xxx series cards in OS4 is going on by 3rd party devs (RV710/HD4350):

http://hdrlab.org.nz/radeonhd-chip-radeon-hd-4350-pci-vga-output-is-working/
http://hdrlab.org.nz/radeonhd-driver-hardware-compatibility/