Hello,
The F4 is an incredible camera.
If you get an F4e and the multi-function back, it will do EVERYTHING the F5 will do AND matrix meter your AI / AIS lenses. You can also switch to one of the smaller battery pack configurations to make it a smaller, lighter camera than the F5.
If you've got an F5, then you don't have to worry about fast AF - just use the F5 for those shots. Also, if you use AF-S lenses the difference between an F4 and F5 / F100 is negligible.
EVERY F100 / F5 FAN on this forum is going to shout BLASPHEMY!!!, but I've done the tests side by side and the facts are the facts.
As already stated, the n90 is not in the same league as the F4/F100/F5. Take it off your list for consideration. After using an F5, you will be sorely dissapointed.
The price F4's are going for now is insane. Get two.