What's It Look Like?
The new Liberty City is not a direct, block for block, replica, but it resembles a striking resemblance to New York City. It is set in modern day, somewhere around 2006 or 2007. The city has 4 of the 5 boroughs of New York and parts of New Jersey. Staten Island is not in the game because Rockstar believed it would not be fun there. Also, Alderney is the largest of the 5 boroughs. The statue of Liberty is in the game and you can go to it by swimming or by boat.
The Five Boroughs
Real Borough
1. Bronx
2. Manhattan
3.Queens
4. Brooklyn
5. New Jersey
GTA4 Borough
1. Bohan
2. Algonquin
3. Dukes
4. Broker
5. Alderny
Broker is Liberty City's version of Brooklyn. Niko begins his journey with his cousin Roman. Roman and Niko run a Taxi Depot there.
Algonquin
Algonquin is Liberty City's version of Manhattan. This has many historic sites and buildings such as Times Square and the Empire State Building. This most likely will be the second borough Niko visits.
Alderney
Alderney is Liberty City's version of part of New Jersey. Not much is known on this borough, but it is said to be the largest borrough of GTA IV. Make sense, considering it is New Jersey, and not a borough like Queens or Manhattan.
Size of Liberty City
There has been much speculation about how large Liberty City is. Magazine articles originally stated it was smaller than San Andreas, now some say it is slightly bigger. Rockstar Games have seemingly said the game is not as big in terms of land mass as San Andreas, however everything is much more detailed, and there'll be no wasted open spaces like the desert.
This is from Game Informer in May 2007: