The 4S is the 5th Generation iPhone. Not the "iPhone 5." The 6th Generation iPhone could be called iPhone 5, however, I do not believe it will. It will probably be called iPhone.
Kind of like the iPod touch or Nano. Which did not have numbers following.
But you said the 4 stood for 4th generation. So what does 4 the stand for now in the "5th generation" phone??
Why look for logic in these things who cares what they call it, if you turn out to be right who's going to care? For the record I'm going for "the New iPhone" at least that would put an end the these discussions!
Reminds me of Curb:
"GTS? GTS stands for Guaranteed Tremendous Safety." "Does that mean the GT stands for just Guaranteed Tremendous?"