Well, I think 2005 is the closest date for PS3, that is unless the PS2 does extremely GOOD or extremely BAD.
From what I have heard, its looking at a 2005 release date. As for GT4, It will supposedly coincide with the Broadband and HDD releases (mainstream).
According to granturismo.com, GT4 hs been slated by SONY for a Christmas/Jan/Feb 2002/03 launch in Japan...
This is possible, considering GT3 was released in April 2001 in Japan.... Thats almost 2 years between games...