Only rode a F4 once, but loved it. Very quick. Faster than my 95 F3, but look at the HP difference and you can figure that out.
F2 and F3 are fairly interchangable, but the F4 is it's own bike. F3s are all carbed, but there is a F4 injected version.
If I had the money, I would move up from my F3 to an F4.
I was wondering if anyone has a F4i with a Cobin seat? Does it improve the comfort enough to be worth it? Does it raise the seat height any? I am only 5'6 so every inch of seat height makes a difference.