Excerpt: Just starting out with a nicer camera and wanted to try a better lens. I wish I had gone with the 2.8 instead of the 4 but cost was a driving factor for me. I think I will be happy with it - but not thrilled. It is light so that is nice - I may change my mind after playing with it more.
Excerpt: If you don't need the 70-200 f/2.8 L II IS Lens' f/2.8 max aperture, the 70-200 f/4 L IS will give you everything else in a smaller, lighter and less expensive package. Without the f/2.8 aperture, you will need twice as much light to stop subject motion (at f/4) - and you will not be able to create a background as diffusely blurred. But, you also give up a substantial amount of size (still fixed), weight and cost.
Pros: Durable, Fast / accurate auto-focus, Lightweight