The F5 is the clear answer for me (and I bought one so this isn't just lip service). It's 98% of the camera the F6 is and it's 1/4 the price.
The F4 is cheaper and it's also a great camera but the motor drive is slower, the AF is a pale shadow of the F5's, and, by nature, is going to be older and more likely to be closer to needing repair. The only real advantages of the F4 to me are the matrix metering with manual lenses (not a big issue) and the out-of-the-box compatibility with non-AI lenses (also not a big issue). I have a few lenses that were once non-AI but got them AI modified. it's far more convenient since you get full-aperture metering on AI bodies and no impairment of function on non-AI bodies (if you even have any such bodies).