In 99 they changed the cbr 600 line alot. The f1/f2/f3 all had steel frames and swingarms, the f4 and f4i have aluminum ones. They redesigned the bodywork, the engine etc.
Basically the engine is a tweaked/improved version of the f3. In terms of powerwise, the change is not too large. I dont know exact figures for the f4.
One thing to keep in mind, the f3 was made from 95-98 so parts are plentiful, and many parts from an f2 will work. The f4 was made for 2 yrs 99-00, then in 01 they changed to the f4i which is fuel injected and has more sleek bodywork. I am not familiar with parts interchangeablilty between the two.
Good luck if you decide to buy, the cbr line are solid bikes.