That doesn't make sense. If your withholding was less it would have been less on your tax return not adjusted after you filed. You got exactly 4,000 less? Any credits or payments on your return for 4,000? Did you claim too much Recovery Rebate credit for the Stimulus on 1040 line 30? Check over your return. Anything look wrong or you don't understand? Maybe someone else here knows what code 9021 refers to.
