If you have the Steam version and have lost your manual backup, you can use a built-in feature to restore of the game's original files.
Unlike newer Rockstar titles (GTA V) which support mod folders or easy hotloading, GTA IV is notoriously fragile. A single incorrect file replacement, a mismatched texture resolution, or a corrupted archive will result in catastrophic failure. gta 4 playerped.rpf backup
Open your Steam Library and right-click on . If you have the Steam version and have
The Ultimate Guide to Grand Theft Auto IV's playerped.rpf: Why and How to Keep a Backup a mismatched texture resolution