Upload & Sell: Off
I agree it could have some use for sports, and not just daytime events. With a camera like the 1DX's great high ISO capability, f/4 is not a significant liability, at least in decently illuminated venues. With Canon's move to FF, there are likely 300/2.8 users who now find it too short. A dedicated 400/4 should be optically better, with faster AF. My guess is it would be priced about 10-15% above the 300/2.8 and would offer a 'budget' option to the 200-400.
But it would still be a niche market, for sure. That said, even though I own a 400 IS v.I, if the supposed 400/4 is not considerably larger than the 400 DO, it could be very appealing. However, it would probably be $2K more than I could get for my lens, so switching really wouldn't be economically sensible.
As to 400/4 vs. 500/5.6... I think it depends on where Canon management sees the priority. Is it news/sports/PJ type applications, or wildlife photography. One argument could be that 400/4 + 1.4x TC is a solution (though of course unpopular here) as it would appease one market and perhaps be 'good enough' for a significant percentage of the other. But a 500/5.6 likely would not appeal to the former, and of course you can't add a 'negative TC,' thus resulting in a more limited appeal. It's interesting to see though, that Pentax recently announced a 560 f/5.6...