Yeap, that little camera keeps surprising me how good it is!
As you can see on my signature, I do have 2 F4: a F4 with the small MB-20 grip for 4 AAs and the larger F4S which uses 6 AAs. Often batteries that are still good on the F90X, will be dead on the F4. It uses a lot of power!
The AF on the F4 is slower, but there is little that the F4 can do that can't be done on the F90X.
The main reason for the F4 is its system expandability. I mean, there are dozens of different screens made to it, the different grips and the many other accessories made for it, turns that camera into a Professional tool.
The F4 is easier to operate in Manual for someone coming from the full manual K1000: you have a shutter speeds dial and the aperture is change with a ring on the lenses, but you have an 100% coverage viewfinder and a full readout of all parameters. The F90X uses a system of button plus a main dial to change values. The F4 has readouts on both the top and bottom of the screen, while the F90X puts everything on just one LCD on the bottom. Both shows you the number of frames taken, the exposure mode and a digital scale in Manual.
Both cameras have a multi-function back available for them. The F4 uses the MF-23 and the F90X uses the MF-26. I own both and they are a bit fiddle to operate. The F90X one is slightly easier to work with as it uses the Main Dial to change values. Both allow a series of functions less often used.
The one for the F4 is larger and has some very nice features like imprinting the shutter speeds/aperture value in-between the frames on film.
Above all, the F4 in any configuration is heavier and larger than a F90X.
About costs. I don't know the American market, but in the UK, you can often see a nice sample of a F90X with the MB-10 going for around £45-50 and the F4 goes for around £125-160 depending on condition.
Finally, the F90X AF motor is said to be one of strongest in any Nikon camera and only the F5 surpasses it in that field.
Have a nice holiday!