I shot F4's for many years every day in every kind of weather. The F4's ran without trouble after the LCD screen in my F100 froze in the mountains of New Hampshire. I never worked in very cold with the F100 again. The F4 is arguably the best tripod camera there is, with every setting visible without having to start the thing up to read it from a screen.
If you are using AiS lenses go with the F4 over the F100, which has much worse compatibility. The F4 gives all metering functions with about any lens that can go on it. The height of backward compatibility. True, some of the newest lenses won't work, but if you're not going to use the latest AF zooms or VR things I'd vote for the F4. It won't let you down.
Of my three F4's only one ever had problems, it came to me very well used, and I shot it hard, daily, for four years. I had a shutter blade develop a crack and it was repaired and the camera continues to run trouble free. I can't begin to guess how much film has gone through that body, I must have put 500 rolls though it a year, and it had a similar working life before I got it, it was at least 10 years old then.
I don't know the F6 well, but wouldn't hesitate to say go for the F4. A student of mine got an F4 about a year ago, he absolutely loves it- and that from someone who grew up reading everything from an LCD screen!