GTA IV, as a game, has a set list of vehicles related directly to the game. GTA IV only treats police vehicles as what it has defined. So, while you can add a virtually infinite amount of cars, you still have to define some parameters (such as in-game name) as a vehicle that already exists. The police vehicles in GTA IV include: fbi, police, police2, police3, police4, polpatriot, pstockade, noose, and nstockade.