I had the f4 for a while at the end of the summer last year. If you are using it mostly for wakeboarding or other outdoor, bright and sunny type stuff, you'll never need the f2.8. I used my f4 when we went wakeboarding and that was pretty much it. In the fall I decided I wanted to shoot some basketball and soccer, so I upgraded to the f2.8.
Bottom line... outdoor bright and sunny stuff... don't need a 2.8 and really don't need the IS. Indoor or low light sports, get the 2.8, indoor portraits or other slow shutter speed stuff, get the IS. My 2.8 is not the IS, since I shoot 90% sports and my shutter never really falls low enough for IS to matter, if it does, I have a tripod.
On the other hand... if you have the money go for the big boy and never have lens envy!