GTA IV redesigned Liberty City for the RAGE engine era. Denser, grittier, more mature than GTA 3's rendition. The map that kicked off the modern Rockstar tradition.

What changed from GTA 3

RAGE engine - featuring new physics, new lighting, increased pedestrian density. Reimagined boroughs: Algonquin (Manhattan), Broker (Brooklyn), Bohan (Bronx), Dukes (Queens), Alderney (New Jersey).

Smaller in geographic size than GTA SA, but far denser - a pattern GTA V and GTA 6 follow.