Are the last four digits in a field separate from the full SSN?
If so, Excel is treating this as a number instead of text.
Are you exporting from a text (or text calculation) field in FileMaker?
If so, try including a single quote before the 4 exported digits to get Excel to treat the data as text instead of a number.
You are the man.
If I change the calculation field to "text" instead on "number" it WORKS.