I'm going to list some other advantages of the 300 f/4 besides the aperture:


1. Better bokeh. Bokeh on the 100-400 can be terrible.


2. Slide-out lens hood. It's convenient and easy to use, unlike the 100-400's bulky, easy-to-scratch hood.


3. the 100-400 isn't sharp until 250mm.


4. the 300 f/4 is as sharp (if not sharper) at f/4 as the 100-400 at 300mm f/5.6.


5. Buying the 300 f/4 with extender gives you both the fast 300 f/4 and a very comparable 420mm f/5.6. Then get a 70-200 f/4 for better coverage of shorter focal lengths.


6. 300 f/4 is $500 cheaper! Come on, that's an advantage for those of us who don't have a money tree in our backyards!