Birding Top 500 Counter
Canon EF 70-200mm f4 L I.S. USM Telephoto Lens
Canon 70-200mm f4 I.S.
4-stop Image Stabilizer
f/4 through zoom range
Fluorite and UD lens elements
Environmental seals
Super Spectra coatings
Soft case and lens hood
Construction  20 elements in 15 groups
Closest Focusing  1.2m
Filter Attachment Size  67mm
Hood  ET-74 (included) 
Weight  760g
Max Dia. x Length  76 x 172mm

O.K. hands up, the Canon 70-200mm f4 I.S. is not a bird photography lens as such and I can hardly recommend its purchase as a mainstay lens for the bird photographer but sometimes you just need something lightweight and has great 'pointability' for other things in life. The fact that it is a zoom lens is also a good bonus for someone such as myself who relies upon prime lenses.

My main reason for buying this lens was for aviation photography when you're dealing with some very large aircraft and aircraft close-up. But after a few months I have grown to appreciate what this lens can do in other areas of photography, notably larger wildlife that can spring up when you're just out and about minding your own business i.e. the deer or fox that suddenly appears in front of you. Fair enough, I have my Canon 300mm f2.8 for the most part, but that is often encumbered by teleconverters so the focal length is a bit long for many situations and it is just too bulky to spring into action discreetly compared to a tiny lens like the 70-200mm. In addition, with a close focus distance of 1.2m, it's not too shabby as a butterfly lens.

So why the Canon 70-200mm f4 lens as against the Canon 70-200mm f2.8? It is cheaper  but not by a huge amount.

Plenty of Canon photographers go through an exhaustive decision process in choosing between the 70-200mm f4 and f2.8 I.S lens, myself included. My own decision was eventually made up by the fact that the lens would generally be used in good light and the addition of teleconverters is not likely for it's intended purpose i.e. large aircraft and display formations. The new 4 stop Image stabilisation is a nice touch but wasn't really a deciding factor.

The 70-200mm f4 lens has a consistently good reputation amongst users, which is not always something that the 70-200mm f2.8 has... although fine examples of that lens abound and produce stunning results.
Again, as mentioned before, another very obvious benefit of the 70-200mm f4 L I.S. USM is its weight and compact nature compared to the f2.8. version Along with the sheer ease at which you can lift the camera and lens up to a subject, it is also a great benefit in use from packed crowds at an airshow (you're unlikely to leave the person next to you with concussion!)

A big subject up close and stuck in a crowd, the 70-200 f4 comes into its own.