I would like a less expensive 400mm f/4.

I am with you 100% on this, I have the 300 f4 IS and it is often not nearly enough focal length even with the 1.4x attached and on a 50D. This is especially true when ever I am photographing birds. But, Canon will not be producing the lens you desire, period. They hit a lot of what you wanted with the upcoming 200-400mm f4 L IS w/ 1.4x. The only problem is it will likely be priced 4-5x what you want to pay ($ 1800). I’m not sure that your price is realistic given that any 400mm f4 would cost the same and be about the size of a 300mm f2.8.

Hopefully Sigma sees how well they did with their 120-300mm f2.8 OS and releases a sister lens possibly a 160-400 f4 OS (still 2.5x zoom range). These 2 lenses would be about the same cost and weight: 3kg and $ 3200 but the latter would provide extra focal range for those who need it. Before anybody says just get the 120-300mm and use a 1.4x converter any converter decreases the quality of a lens noticeably, especially the 2x variety. A bare 160-400mm f4 would beat a 120-300mm f2.8 + 1.4x converter handily. I am often looking for even more reach that 400mm and would likely be using a 400mm + 1.4x converter a lot (which would easily beat the 120-300mm + 2x). If such a lens existed it would be part of my budget next year for sure!

jay