I prefer the F4 to the F3. I only use manual focus lenses, with the exception of a lone 50mm f/1.8D. I'm normally able to manually focus just fine with my preferred E screen, but if I get stumped, the electronic rangefinder works in surprisingly low light. The three LEDs of the electronic rangefinder bother some people; I like it fine. The backlit display of the F3 is a joke; the F4's is much better.
My F4 has the MB-20 grip; alkaline batteries don't seem to last long; lithiums are better. I find the hand grip very comfortable, but again, there are those that hate it. I'd say that the build quality of the F4 is inferior to the F3; it uses more plastic. Personally, this makes me less afraid to use it in the rain and dirt.
LTD_4224_001 by LJ Slater, on Flickr