Is the AF system too slow? In that case, the F5 will be better and you will have excellent metering should you decide to point and shoot. Moving from an F4 to the N80 (back-up) in a hurry will not be as efficient as with an F5, especially if you have to switch from setting the aperture with the rear command-dial to setting it via shutter-priority (as G lenses don't work fully on the F4). The N80 will work as a lightweight F5 should you get any more of the G lenses in future, and it won't eat as many batteries. I would get the F4 only if you love manual focus lenses.
Jonathan.