4xxx Radeon's can mine just fine, but it's not economically feasible at this time (high power consumption, low hash rate)
That is also wrong. I have a 4870 as the OP, and standard I got ~82 MH/s with poclbm -w 256 -f 60.
Since I'm also using the computer normally while mining, the only real cost is that of idle vs full load, which is 80w at stock speeds.
By overclocking core and underclocking memory to 830/190 I get to 94.5 MH/s, giving an 85w effective draw during mining, but overall 30w lower due to memory underclock.
So it is most definitely profitable.
4870 is 150W at stock, which is why I stopped mining on mine, as I was paying more for electricity than I was getting back from BTC. Not sure if it's profitable now BTC value have gone up, as the difficulty has also gone up.