It strikes me as very unusual that an F4S would have such a problem. Most working professionals do look after and service their equipment, even if it is dented, scraped, scuffed, bent or busted but still working. But the F4S is a workhorse, so how then did this shutter problem eventuate? By somebody poking their finger into the shutter? I would be inclined to give it a very thorough visual and technical examination (Nikon, like Canon, has diagnostic devices that plug into the camera to read-off various statistics that can be quite revealing and quite contradictory to the oh-so-sweet salesman's pitch. Go for it.