Box 4 — Social Security tax withheld
Employee Social Security tax withheld—normally 6.2% of Box 3 wages.
What it means
For a single employer, Box 4 should equal 6.2% of Box 3 (within rounding). If you changed jobs and overpaid Social Security across employers, claim the excess as a credit on your return. This amount is not federal income-tax withholding.
Related glossary: fica