If you have the STEAM copy of GTAIV installed, you can simply go to your "Library", Right click on GTAIV, Go to "properties" and select the tab "Local Files" and at the very bottom click on "Verify Integrity of Game Cache" It will re-download the files you need creating a "fresh" install of the game files you have changed or deleted. Saves the time of re-downloading the whole game.
In the case you don't have the STEAM version, feel free to ignore this post. Hope this helps!