f4 has a much faster flash synch speed than the f3. Of course: depending on what you want out of it, that might matter a lot or nothing.
f4 also has an inbuilt diopter adjust - invaluable for tired eyes like mine - and the metering is spot, cw and matrix. It will take all modern lenses including G-type, only doesn't do VR. The f3 has screw-in adjusts and the meter is just cw. You can only use standard non-AF or AF non-G lenses. Which is the majority of them anyway!
The f3 is much lighter than the "traditional" f4s. On the other hand, if you get a f4 with the small 4-AA MB20 grip and the multi-function data back MF23, it's not that much heavier than a f3 with motor drive and it likely is less bulky as well as much more egonomic and can do a lot of extra things courtesy of the back. I particularly like the imprint of speed and aperture data between the frames.
Oh, I forgot: the f4 has a very well dampened mirror slap while the f3 is a bit more f2-ish (if that makes any sense - only way I know how to explain it...). Basically:
I can shoot the f4 handheld at lower shutter speeds than any other camera except the ZM rangefinder.
I use a f4 with mb20 and mf23, as well as f2as with a md3. Used a f3 back in the 80s but never quite liked the slow flash synch and the general feel of it. This is not to say it is a bad camera! Just not my cuppa.