There are probably a few more differences than that, this being Porsche, but yes, that's essentially what the S is. The S and 4S, however, weren't released until '96 and '97, which puts them out of the scope of the rest of the NFS cars, which are all 1994/1995 models.
Personally, I would prefer to keep the NFS release to just the two Carreras, with the Turbo/Ss in a separate download like we did for the Diablo SE30 Jota. I'm not really in love with the idea of distributing extraneous models or upgraded fantasy S1 type stuff in the core packs. Primarily a matter of staying on-theme with our "official" cars, but also being mindful of download file size.
I fully support all the other stuff as standalone downloads, though, and I'm willing to do an S/4S if Dan wants to model them. Same goes for any variants
@Kyuubeey has in mind. (Arch, one really useful thing would be replacement physics for a '94 FD to replace Kunos' Spirit R, and maybe a '94 Supra. As it stands now those two aren't the best fit, and you're the JDM wizard.)