Will not be a 4000 series (at least not in the off-the-shelf kind of way). It'll be a heavily customized GPU, as that is how consoles' GPUs are made. Earliest dev-kit rumors suggested a 4000-series card, but personally, I don't see that being the case. Even if it was true, it'd be leaps and bounds ahead of what the current-gen systems can do. I do expect some nice DX11 features, although the card will use OpenGL instead of DX11.
We did see gorgeous global illumination in the Zelda tech demo. Anyone have a video of a game on the Xbox360 or PS3 with similar levels of global illumination and overall detail? I'm curious, is all.
The tech demos were running on early dev kits (probably the ones which were reported as having a HD4000 series card in them), and Nintendo have a history of showing tech demos which get completely outclassed by real games, so I wouldn't take the demos to be entirely accurate representations of the system. That said, I personally believe they look noticeably better than anything I've seen running in real time on the PS3/XBox360, so I'm expecting pretty nice things once we get to see real games running on final hardware.