It depends a lot on which lenses you will be using, and whether you are looking for a pro type body at all cost.
If you need pro quality with G AFS and VR lenses, then definitely get the F5, or as others have said, the F100. If your lenses are AF only, and you do not need blazing fast focusing or frame rates, then the F4 will probably do fine, and will also be smaller so will take less space in a bag. The ultimate is the F6, of course, which is still very expensive and even if it could take a beating, would be hard to justify beating up given its price. If you know you are going to beat up the camera, then an F4 will at least be cheaper to replace. As an aside, the FA will also give matrix metering with AI/AIS lenses, and is in itself a lovely camera, but not AF of course.
How many rolls do you shoot a year? If like me less than 100, I think the F100 is actually a very nice camera that has most of the F5's good qualities without the bulk. It was and still remains a very popular camera. While the F5 is as rugged as any camera ever made, they can and do give problems from time to time. Mine is currently in for shutter repair/replacement. But there is nothing the F100 won't do in its absence. I have three of them, which allows me to load three different types of film or alternate bodies while reloading. In handling the F100 is much like my D700 and D800, so there is less of a mind-shift going from analogue to the other side and back. The same would of course apply to the F6, which is in essence very similar to a D2-series. It is hard to gauge how harshly you intend to treat the camera. An F100 will take a fair bit of roughing and toughing, but like all cameras it is not meant to be dropped, inundated or thrown at passers-by. I would defintely take it on a hike, for instance. If you take care while loading film and when handling the camera with lenses on, the mount and chassis are strong enough for the heavier lenses (provided you don't hang your 400/2.8 off the mount only), and the rest of the bits and pieces should not break that easily. The earlier F100s came with a plastic rewind fork that some have reported to be prone to breakage. Either they are very rough in changing cassettes, or the camera has done 5 000 rolls. Anyway, it can be replaced with a metal fork. The back latch can maybe also break, and can likewise be replaced by a metal part. Shutters will last around 100k to 200k actuations on average, so if you buy a well-used F5 or F100, the shutter is the most likely area of concern. I tend to look for "soccer mom" cameras in the used market, and try to avoid those that saw active duty with photojournalism and the likes, but sometimes you have to take what you can get. Sometimes that even means buying a broken camera and fixing it.
I forgot to add: The F90 and its variants, and maybe even the F801, might also be just fine for you, since they will meter with AI/AIS lenses. I am not 100% sure whether it is matrix metering, but I am sure you can look that up. The main advantage of the F100 over F90X is forward lens compatibility (G, AF-S, VR) and generally better focus, metering and ergonomics. F90's and especially F801's are so cheap that you can buy four or five of them for the price of one F4, so wrecking one isn't much to cry about.