Try this:
1) Create a hidden field ("hiddenField") to store the SSN value
2) In your SSN input field under the enter event (JavaScript) put:
this.rawValue=hiddenField.rawValue;
3) In the same field under the exit event put:
hiddenField.rawValue=this.rawValue;
this.rawValue="*";
for (var i=0;i<hiddenField.rawValue.length;i++){
if (hiddenField.rawValue.length>i+4)
this.rawValue+="*";
else
this.rawValue+=hiddenField.rawValue.substr(i,1);
}
Kyle