The F4 I would consider more of an upgrade to the N8008/8008s, since it was designed around the same time the 8008 was designed. AF is slower than the N90, and some of the special features that the N90 has aren't on the F4, like the FP flash synch, distance calculations in TTL flash, etc. The F4 is built better, though. LCD bleed is a common problem. Out of the five F4's I've owned, three have had LCD bleed issues. One was major, but was only in the DP-20 prism, so I was able to change the prism out for a good one. The other two had bleed in both the DP-20 LCD and in the body LCD. My current body is one of those two. Doesn't have much, but it's there. I'd recommend getting one with the MB-20 battery grip, instead of the MB-21. Run the Energizer lithiums in it for a speed increase, and for a lighter camera.
I have discovered that the F4 can be stored uncocked. Fire the shutter on "T", turn off the camera, then move the shutter speed dial to a different setting. The mirror won't drop back down, but the shutter will close without cocking and advancing to the next frame. To awaken the camera, turn it on, then depress the shutter release. It'll advance the film and the mirror will drop down.
-J