Redacting Partial Social Security Number so only last 4 show

I find this feature works occassionally, such as for account numbers, but I can't seem to get a partial redaction to work. For example, if the SS# is 123-45-6789  i would put into the find text field within acrobat redaction tool "123-45" and it would pull up the entire SS#.  

Does anyone know how to do this?

Thanks!