TexPhoto said:
The 300mm f4 IS is ...So sharp you can crop to the view of a 400mm and have a better photo than with the 100-400mm
The 300/4 is a little sharper, but not
that much sharper - comparing the 300/4 wide open to the 100-400 @ 400mm wide open, they deliver about the same resolution in both center and borders. The 300/4 stopped down to f/5.6 is a bit sharper in the centers, but about the same at the borders.
I agree that the 300/4 is a great lens, but really, it comes down to the focal length you need/want. I wouldn't get a lens planning to crop most of the resulting images. Also worth considering the intended subject(s) - for example, if you're shooting birds, most likely even 400mm will on APS-C will require some cropping, 300mm would require more and a correspondingly greater hit on IQ (which is why I sold my 300/4 and bought a 100-400 instead). 400mm is a break-point - a longer lens will cost you
significantly more (or you'll take a hit on IQ with less expensive 3rd party lenses).