Obscuring 4 digit number field with asterisks

I have seen similarly asked questions about obscuring ssn fields, but my question is about osbcuring a PIN field. I'm creating an interactive enrollment form that will auto populate other fields by means of scripting based on the four digit PIN the user enters at the beginning of the form. The 4 digit PIN serves as the trigger. Once the PIN is entered, it shows the user what his current benefit enrollment elections are and then he can continue to fill out the rest of the form for the new year.

I would like for the entry of that PIN to be replaced with **** as the user types or changes to **** once the user presses the enter key.  I'm fine with either action. I'm still very new to actions in Acrobat and Javascript, so please dumb-down your answer for me. Thanks mucho!