lace the m4Super2 file into "...\SteamApps\Common\Left 4 Dead\left4dead\Materials\Models\v_models\weapons"
If you do not have a "weapons" folder in your "v_models" folder, simply create one.

Using Notepad++, open pak01_dir.vpk in your left4dead directory.
CTRL+F and type m4Super
When you found it change it to "m4Super2" and save

So it should look like this

$baseTexture "models\v_models\Weapons\autoshot/m4Super2"

Now play the game