I'm definitely hoping that it gets replaced with a 400mm f4 IS lens at an affordable price.
I realise that both the 400mm f2.8 and 500mm f4 are really expensive, but the 300mm f4 is under £1000, so shurely a 400mm f4 could be made that isn't that expensive.
A 400 f/4 would be quite a bit larger, heavier and more expensive than a 300 f/4. Just look at the 300 2.8 compared to the 400 2.8! If it became too close to the 300 2.8 IS then people might just stick to using a 300 2.8 IS + 1.4x TC so I'm guessing that is why it's not been done.