so a 400 f/4 probably won't be small or cheap ... but can't we get a 500 f/5.6 IS? the sigma superzooms that hit this focal length (150-500) aren't huge or hugely expensive.
The Sigma xxx-500mm zooms aren't 500mm f/5.6, they are 500mm f/6.3. Sure, that doesn't sound like much...but 500/5.6 = 89mm, whereas 500/6.3 = 79mm. So, that innocent-sounding 1/3-stop translates to some elements needing to be 27% larger in area (which, of course, is the exact reason those less expensive Sigma zooms only open up to f/6.3). I wouldn't expect Canon to start releasing f/6.3 max aperture lenses anytime soon, and likewise, I expect high cost barrier to get over 420mm with a Canon lens that can AF on most bodies will remain in place.
still, the Sigma 500 f/4.5 is only $5K, and so a Sigma 500 f/5.6 would be, in my book at least, probably a reasonably-priced lens (especially for the length). Frankly though, I'd be happy with a true 480mm f/5.6, or 450mm f/5.6 ... anything to get out of the 400-420 range