Canon 70-300L IS USM Vs Canon 70-200L F4 IS

I use my 70-300mm L IS USM at all times of the day but have used quite a bit for shooting wildlife at dusk. On a 450D I have had no problem with autofocus, IQ, sharpness or shutter speed on deer or other mammals. The only time I have had too slow a shutter speed is BIF in low light as I cannot raise ISO above 1600. However I get no better a shot at these times with my 300mm L f/4. I have found the difference between f/4 and f/5.6 has little to no effect.

Regards
Gelbvieh