That price is a bit high for an F4 body.  Here in the US, market on a
*mint* F4/F4s is about $600, and the price drops rapidly with any
visible wear or other defects.  A "very good" condition F4/F4s should
cost you in the neighborhood of $400-450, and an "excellent" condition
body should run you right at about $500.

Things I'd look for:

* There should be a threaded ring around the eyepiece, to which a
rubberized ring should be attached.  If the rubberized screw-on ring
is missing, that's OK - you can get a replacement.  It helps protect
the viewfinder shutter and eyepiece glass, so it should always be left
on.  If the threads are snapped off (and it's a common flaw), you
won't be able to mount diopter adapters or a right-angle adapter to
the viewfinder.

* The DOF preview button (silver, to the right of the lens mount when
holding the camera normally) should be firm to push, but should not
bind or make loud "squacking" noises - both indicate wear or damage to
the lens diaphragm coupling mechanism.  Make sure all dials and other
controls operate freely without binding.

* The LCD displays in the viewfinder should be clear.  There should be
no cracks, blobs, or missing character elements.  A green LED
backlight should come on when the shutter release is partially
depressed.  If not, check that the switch under the shutter speed dial
is rotated to the right, with a red dot showing.  If the backlight is
still not functional, you've got electronics problems somewhere.

* Take off the grip (there's a locking lug on the bottom) and examine
the battery packs and battery contacts.  If it's the small 4-cell
MB-20 grip, check that all the springs and contacts are clean and
corrosion-free.  If it's the MB-21 power pack, remove the 3-cell grip
and slide out the 3-cell carrier in the lower battery bay.  Again,
check all contacts.  The body has two spring-loaded battery contacts
in the grip housing next to the locator pins - make sure they depress
and spring back properly, otherwise you won't be able to use the MB-20
grip.  Any evidence of battery leakage or corrosion should be a
warning.

* Of course, check that all glass, mirrors, prism, etc. are in good
condition and undamaged.  Take off the viewfinder (the release is to
the left - push it toward the housing and carefully pull the
viewfinder off to the rear) and inspect the focusing screen.  Remove
the screen if you like by getting your thumbnail under the rear edge
and lifting.

* Be sure that the film cartridge spool works freely - flip up the
rewind lever and wind it by hand to make sure it's not bent or bound.
The film door should unlock when the release on the takeup spool is
pushed in the direction of the arrow, and the winding lever is pulled
upwards to release the spring-latch.  Check inside the body for
excessive wear marks, fingerprints or foreign matter on the shutter
leaves, loose floppy bits, etc.  When closed, the film door should be
flat with the body frame all the way across - a bowed door is a
potential light leak and a sign that the camera has been dropped.

* Remove the body lens-mount cover and check that the mount's in good
condition, the diaphragm coupling lever on the left side isn't
bent/broken, the aperture index tab on the top of the mount isn't
broken (it should flip up & down, though), the electronic lens
contacts are clean/undamaged, and the mirror is in good shape.  The
indexing lever at the bottom of the mount should be spring-loaded and
move freely.  If possible, mount an autofocus lens to the camera, put
the focus selector in the "S" position, and engage the autofocus by
partially depressing the shutter release.  The autofocus motor makes a
high-pitched whine, but should not grind or rattle.

* Put the camera in C-h (continuous high) winder mode, switch focus to
"M" (manual) with the selector below the lens-release button, set the
shutter speed to 250, and the exposure mode to "M" (selector under the
exposure comp. dial).  Manually set the ISO dial to some non-DX value
and hold down the shutter release.  The camera should cycle at about 4
fps, and there should be no untoward grinding, straining, unusually
slow cycling times, or other ugly noises - the mechanism in this
camera is rather quiet - any loud obnoxious noises should be suspect.
If the camera has the MB-21 pack, make sure that the vertical-grip
shutter release works as well.

It's not unusual, particularly on earlier serial numbered bodies, for
the markings on the ring around the shutter release to wear -
typically the C-s and timer symbols.  Likewise, I wouldn't be worried
too much about wear marks on corners or the rubberized portions.  If
the camera's in good mechanical shape, it'll last you for a long
while.  The body's a tank, so it'll take a certain amount of
mistreatment in stride.  That also means that if there's any damage
evident, the body's really been ridden pretty hard and should probably
be avoided.

Good luck.