The Nikon S4 with the 35/2.5 in 1.8 style barrel
Basically this, and the S3 style finder, are the only differences between the S4 and SP. All of the curtains were cloth, and it was not available in black finish.
Nippon Kogaku ran into a problem with the S4, though. EPOI was the sole distributer of Nikon products in the US. Joe Erenreich, the "E" in EPOI (Erenreich Phot-Optical Industries), refused to import it into the US. He felt that the S4 would not add to overall Nikon sales, but would only take sales away from the S3. He also felt that the future was the Nikon F, and wanted all resources channeled to it.
As a result, the S4 was not sold in the US, except through the Post Exchange system on military bases, and was also sold in Japan, and possibly South America. As a result, only 5898 of them were manufactured. The majority of the S4 cameras have serial numbers 65xxxxx, but a few are numbered 63xxxx, like S3s.