If you think about it, the new iPhone will be 3rd generation, not 4th.
iPhone - 1st Generation
iPhone 3G and 3GS - 2nd Generation
iPhone (New) 3rd Generation
There was only a hardware boost in the 3G to 3GS.
Actually it was quite the opposite if you go by the names in the firmware.
2G - iPhone 1,1
3G - iPhone 1,2
3GS - iPhone 2,1
new iPhone - iPhone 3,1
So you see, the 2G and 3G were actually the same "generation". But this is most likely related to the processor, not any sort of "generation".