actually the 100-400 can do f4.5 - that is (was) the valid point
The difference is that at f4.5 you're shooting between 100mm and 130mm, which is much less zoom than 300mm. you're shooting at f5.0 from 130mm-260mm, and from 260mm-400mm you're shooting at f5.6. the 100-400 can never do f4.
If you need speed and close focusing, get the 300 f4, if you need zoom versatility get the zoom.
the 300 f4 is also sharper @ 300mm than the 100-400 is at any focal length at any aperture.