Setting up to show only last 4 digits on ss or credit card field using input mask Results

I have a form that gathers employee social security numbers. I am using access 2007. Is there a way to [after info typed in] only display the last for digits of the SS#? so 444-44-4444 would show as ***-**-4444? I have done a search but not finding anything. I could have sworn i saw this somewhere before, but cant find it.

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