There's already five pretty decent lenses covering that focal length: the f/2.8L IS II, the f/4 DO IS, the f/5.6L, the 100–400mm f/4.5–5.6L IS, and the 200–400mm f/4L IS. Of these lenses, there are three that offer f/4 IS and thus another one would be serious competition for them all.
There are 11 pretty decent EF lenses covering the 200mm focal length, and of those there are four that offer f/4 IS. I don't think Canon is too converned with multiple lenses covering a given focal length. Generally, more choices are a good thing - as long as Canon sets a price that provides a return on investment for a given number of units sold, it's a win.
It's also worth noting that two of those three lenses that provide 400mm f/4 IS are quite new, and deliver substantially better optical performance than the 400/4 DO. Given that, there is quite likely a market for a 400mm f/4 IS lens that delivers optical performance on par with the recent MkII supertele lenses as a price that's around that of the current 400/4 DO, i.e. $4K cheaper than the 400/2.8 II or 200-400/4+1.4x.