I'm using TheSSN and DisplaySSN as my two fields in my example just exchange with your field names.
You need two fields, one that hold the SS number complete (TheSSN), then a second calculation field (DisplaySSN) that show the last four digits of the SSN.
Calculation would be "XXX-XX-"&right(TheSSN;4)
Put both fields on the layout. For the calculation field, uncheck field entry from the inspector. Make both fields the same size and then place the calculation field on top of the ssn field. Make sure that the calculation field is on top.
That worked beautifully! Thank you so much! Much easier than what I was making it out to be.
I am glad to see this. I also do this with telephone numbers, I enter/edit a 10 digit telephone number, numbers only but display by whatever way I choose. Great technique since true masking is not provided in Filemaker.