Mask all but last 4 of merge field

Content

I'm using a transactional survey, comprised mostly of merge fields, and business rules to send receipt for some of our forms.  In some of the forms we request sensitive information, e.g. account numbers.  I see there's a way for merge fields to only show the first x amount of characters, but I'd prefer to only show the last 4 characters.  I'm basically looking for similar functionality as Incident Thread Masking to take place in my message templates and mailings, specifically targeting the merge fields.  Anyone know if this is possible?

Thanks!