Wait, so you *want* to shoot blurred images?
The f4 lenses are sharper than the v1.0 of the f2.8 lenses, but the new 70-200 f2.8L II has closed the gap. Of course, if you have the budget for it, I would get the new f2.8 IS, but if not, I'd get the f4 IS, as it will give you up to a 3 stop advantage over the non-IS lens.