In a large mixing bowl, cream the cream cheese with an electric mixer.
Add the skim milk and beat until thoroughly combined.
Add the powdered sugar-free Jello chocolate pudding mix and continue mixing.
When the mixture is well combined, fold in 3 cups of sugar-free Cool Whip.
Pour mixture into chocolate crumb crust.
Refrigerate for at least 3 hours to allow the pie to set.
Just before serving, spread the remaining 1 cup of cool whip over the top of the pie.
Garnish with grated chocolate, if desired.