I would assume that, given the Canon EF 400mm f/4 DO L IS lens runs for about $6000 (and I think lists for $6999), a non-DO EF 400mm f/4 L IS lens, especially if its got top-notch near-perfect optics like the new 500mm L II and 600mm L II lenses, it would list for a fair bit more than the DO (DO may be smaller, but it does have a definite limit on resolution...*diffractive* optics). Probably looking at $7999 - $9999. I think everyone would like to have a $3800 400/4, but once you get into high resolution, high quality, image-stabilized telephoto optics (which is what Canon excels at)...the differences in price tend to be minor. I'd be surprised to see such a lens for less than about ten grand....I've always been under the impression that the price of the 400 f/4 DO was pushed up because of the diffractive optics, as it is in the same price range as the original 500. Had the new supertelephotos not been significantly more expensive compared to the original versions, I would have expected it to be cheaper than the DO, now it will probably be as much or more though, probably in the same range as the 300 f/2.8 MkII.
When you get to lenses of that caliber, the only real significant cost to design is the optics. Some of those lenses require HUGE elements that have to be perfect across their entire volume, and often require a lot of element groups with a high number of total elements. In the case of the DO lens, there is one very expensive element group (the DO group)...however because of that Canon was able to eliminate a lot of other corrective groups that would have otherwise been necessary...at the cost of maximum resolution (the diffraction is going to etch away at it a little bit.) Given the size that a 400/4 lens would need to be (larger by a fair amount than the DO version), and the optical corrections that would have to be performed to keep IQ at 500/600 L II levels...there are simply going to be more
big, expensive lens elements.
Think 8-10k is too high. Nikon's 200-400 f/4 is ~7k, and Canon's 400 f/2.8 is 11.5k. I would expect the 400 f/4 price to be well below Nikon's excellent 200-400 f/4. Given the ratio between Canon's 2.8 and 4.0 versions of the 300mm (7300 vs. 1400), one might expect that the the 400 f/4 might cost closer to 2-3k. If it's 8-10k, most people might consider saving up a little more for the 400 f/2.8.
Now, its certainly possible for Canon to create a 400/4 that does not have 500/600 L II level IQ...but why? They already have the 400/4 DO to cover that spot, in a much smaller lens body with much lower weight, and is otherwise a superb lens outside of the limit on resolution and IQ (which is minor, at that...look at Art Morris' bird shots with that lens...they are stunning; the notion that IQ is rendered useless by the DO element is largely myth). If Canon creates a 400mm f/4 L IS lens, it seems logical that they would make it a high quality, top-end lens lacking nothing. If its not a DO lens, then they don't have the diffractive optics corrective benefits...so they are going to have to correct optical aberrations another way (i.e. UD and flourite elements, multiple corrective groups, etc.), and that'll increase cost. Personally I think it'll land around $8000, maybe $7500...f/4 @ 400mm isn't quite as much a feat as a 500mm f/4 lens....but its still going to be expensive.