This is an unofficial update for GTA 4 called ILL-GOTTEN GAINS Update: IV. It adds (does not replace) 8 new vehicles that were recently released in the DLC for GTA 5 with the same name. All vehicles travel around the city, but they can also be spawned using a trainer in the Added Car Spawning menu. I am not the author of the models.
Transport list: Transport - author
Albany Virgo - PG1313
Benefactor Stirling GT - PG1313
Coil Brawler - Thundersmacker
Enus Windsor - PG1313
Pegassi Osiris - Thundersmacker
Progen T20 - Thundersmacker
Lampadati Toro - PG1313
Dinka Vindicator - PG1313
Installation:
Run the installer, specify the game folder.
Deleting:
Run the DLC_Uninstall file in the game folder and click "delete".
Copy the files from the backup_files folder in the game folder and add them along the path: GTAIVcommondata.