I have the older 70-200/2.8 IS. When I was thinking of upgrading to the vII version of that lens, a friend of mine suggested I look at the f/4 IS instead. For what I could sell the f/2.8 and what it would cost extra to get the vII of that lens, I could get the f/4 IS on sale, so I did that and kept the f/2.8 v1. I will say, the f/4 is sharper then the 2.8/v1 by a good margin. The AF is more consistent as well. These are the two things I expected from the 2.8/v2, but of course in a much bigger & heavier package than the f/4. I'm happy with the combination I have now, the f/2.8 for those times I really want the most luscious bokeh possible (the v1 in this respect is reported to be better than the vII, which I've heard has a slightly 'harsher' bokeh, which seems to be the only aspect of that lens that wasn't improved over the v1), and the ultra light weight, super sharp photos of the f/4 when maximum shallow dof isn't the primary need.
All this is to say, like everyone else has said so far, I don't see any pressing need to update the 70-200/4.0 IS lens. It's really one of the best performance/price combination lenses in the entire Canon lineup. It's like the old saying, "don't fix it if it ain't broke."