While Los Santos, San Fierro, Las Venturas, Vice City and the original Liberty City were decent approximations of iconic American cities, the debut GTA IV video hints at an incredible level of realism for the PS3 and Xbox 360 iteration - not just in terms of the buildings but also their geographical positioning and sense of scale.
New York will still be referred to as fictional Liberty City but it seems feasible from close examination of the trailer that an aerial view of the environs will match the authentic Google Maps version far closer than the one seen in GTA 3. In many ways it's approaching the whole city thing in the same manner as The Getaway did for London.
What does this mean for the gameplay? Everything points to a less cartoonified visual style and a more mature subject matter with the Eastern European character mentioning people smuggling as one of his past crimes. Then there's the austere orchestral music, a far cry from the funk and hip hop of San Andreas and the 80s ballads of Vice City. Could this be suggestive of a major change? To look at it another way: are we looking at the first 'serious' GTA?