You might be making this more complicated than is necessary, but then you didn't answer why you need the x's. Why isn't the label permanent ("Last 4 SSN Characters"), make the control wide enough for X's and last four, then when record is saved, prepend 1234 with x's and save that. Then no matter which record you move to, it will show X's then 1234.
If not that, there must be a simple way. I think our problem is that we don't have the whole picture.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.