I like the Nikon F4 quite a bit. I made a spotlight for it almost exactly five years ago, which can be viewed here. A couple days ago, at an estate sale, I found a Nikon F4s in great condition. Besides a thin little cosmetic crack on the viewfinder, it seems near-mint!
So, what's the difference between an F4 and an F4s? Turns out, it's the battery pack. When the smaller MB-20 pack is installed on the camera body, it's known as an F4. When the beefier MB-21 pack is attached, the camera becomes an F4s. Besides these swappable battery packs, the cameras themselves are the exact same.
While the F4s setup is larger than the standard F4, and takes an additional two batteries (6 vs. 4), you do get some cool added features. With all that extra battery power, the F4s can shoot at an impressive (for 1988) 5.7 frames per second, quite a bit faster than the 4 fps of the standard F4. Though the F4s is huge for a 35mm camera, the MB-21 does afford it a vertical grip, which makes framing portrait-oriented photographs much more pleasant. I actually prefer the handling of the F4s to the F4, as long as I don't have to carry it around with me very far!
I took my F4s out with me to photograph what it was designed to photograph: sports. My vintage base ball team, the Chicago Salmon, had a game in Valparaiso this past weekend, and I captured some of it on film. Overall I was impressed with the results. As I was in daylight for the entire shoot, the autofocus worked very well. Most of my photographs came out acceptably sharp, and very well exposed. While I don't think the F4s could keep up with faster, more unpredictable sports like soccer or basketball, for baseball it definitely sufficed.