AMD have now announced that they won't be supporting Radeon HD 4000 series and below in Windows 8. Most people will read this and be shocked, but the GPUs are old now and Windows 8 by the time it comes out, will be considered a 2013 release.
Windows should ship with some form of support for legacy Radeon cards, but AMD themselves won't be providing future driver updates for those GPUs. AMD have made this quite clear in their press release:
Also with regards to Windows 8 support for the AMD Radeon HD 2000, 3000, 4000 Series of products; the In-the-box AMD Graphics driver that ships with Windows 8 will include support for the AMD Radeon HD 2000, 3000, and 4000 Series, and it will support the WDDM 1.1 driver level features. The AMD Catalyst driver for Windows 8 will only include support for WDDM 1.2 support products (AMD Radeon HD 5000 and later).