I doubt that AMD cares about their HD4000 series anymore
I'm curious what would lead you to that conclusion. Pure assumption on your part?
I run 2x 4870x2 in Quad CF and I can say with absolute certainty that they work with and benefit greatly from using newer drivers. The BF3 Preview drivers made a clear difference on my hardware. You don't get much newer than that as far as drivers are concerned, which makes me feel that they certainly are continuing driver development.
Take a look at the steam hardware survey:
64%+ of all GPUs currently still in use are DX10-era. DX11 represents less than 29%. 6 out of the top 10 cards are DX10 hardware including two cards from the 48xx series.
AMD isn't going to drop the ball on tons of people who still use these cards. It's WAY too soon. Also, 48xx series is VLIW5 just like the 5xxx series and the 68xx series, supporting these cards is probably very easy.