I started with a 70-200/2.8IS back in December 2007 (version 1, obviously). After several successful rentals, I got my wife the 70-200/4IS in June 2013 during (instant) rebate season. After "borrowing" her 70-200 on several occasions when I didn't need the extra 1.75 pounds for one stop of light, I decided to buy the 70-200/4 about 15 months ago, and use it whenever I don't need 2.8 or IS. I can't say that I've really compared shots super-closely, but I don't notice a difference between the f/4 variations. I certainly don't notice a need to avoid certain zoom/aperture/focus-distance combos; I do avoid shooting my 2.8IS at 200mm wide open if at all possible since it gets rather soft.

(By the way, please don't tell my wife we have three 70-200s. She hasn't noticed yet...which includes the new one being in plain sight for a 7-day cruise...)