Instructions
Download the "Ragdoll Mod" from the GTA Gaming website. This mod is actually just a script that gives players control over the ragdoll features in the game.
Right click on the downloaded file and select "Extract All." Select the installation folder for "Grand Theft Auto IV" as the destination for the files.
Navigate to the GTA-downloads website and download the "GTA IV ASI Loader" program. This file is needed to load the script file for use in the game.
Extract the contents of the "ASI Loader" archive to the folder where "Grand Theft Auto IV" is installed.
Start the game then press the "H" key on your keyboard to enable ragdoll mode. Press the "Spacebar" key to return to normal animated mode. The mod will also make the main character go into ragdoll mode when he receives any kind of damage as long as he is not wearing any armor.