I'm going to speculate a little bit here and say that I think Sony has some "money-spinning" plans for the GT4 franchise.
Initially, they released the Prologue disc to generate some interest and make a few dollars, then comes the GT4 game to make the serious money.
After that will be the "expansion disc" to introduce online game play, to keep the cash registers turning over.
My bet is that in order to play GT4, you will need to have the Prologue disc, and to play the later "expansion-online gaming disc", you will need GT4.
Sony are shrewd operators and know how to corner the market once they are onto a winning formula.
The scary thing is, we can forsee it happening, and yet, we will still continue to buy the games.
As I said, I am purely speculating, but my money's on this occuring.