My 2 cents. I bought the f4 IS before, and didn't think it was worth the price, so I returned it and got the non-IS. I'm much happier with the extra money in my pocket than I was with the IS.
For the price (and even cheaper where I live) of the f4 IS, you can get the sigma 70-200 2.8 OS, which has better build (no weathersealing though), 2.8, tripod collar, and a longer warranty. If I did feel able to spend that much money on this hobby, I'd get the sigma without hesitation.
As for whether or not you need IS, that really is dependent on you. If you shoot still objects in low light, then you'll probably need the IS, however, IS won't help you in any way, shape, or form with moving subjects. As for the difference in sharpness, don't let that be a factor in your decision, because having owned both the IS and non-IS, I can tell you right away there is not much of a difference (at least nowhere near double the price worth), especially if you don't pixel peep at 300%. The main difference really is just the IS.
I'll freely admit that I sometimes wish I had IS, but then again, lack of it hasn't made me miss a shot so far, and I'd rather use a tripod than spend the extra money, since a good tripod is cheaper than the price difference between the 2 lenses anyway.
I wouldn't really consider any of the Canon 70-200 2.8's because of the following:
The non-IS costs more (where I live) than the Sigma OS, and they're about the same in terms of IQ, the Sigma might edge it out slightly, and the Sigma obviously has OS. The Canon IS Mk I costs quite a bit more than the sigma, and IQ is not as good. The Canon IS Mk II is simply out of budget, though it is better than the sigma in every way except warranty.