It’s not clear why you need to make such delicate adjustments to your W-4 unless you have outside sources of income that you were trying to cover for. Normally, the W-4 is set it and forget it.
In any case, start by making an estimated payment on the IRS website, www.irs.gov/payments. You will select “2021 estimated tax payments” from the reason menu. Then in TurboTax, you can list the estimated payment in the section titled “estimates and other taxes paid” which is on the page for deductions and credits.
Any payment made by January 15 will be counted as a fourth-quarter payment. TurboTax may offer to calculate a penalty for you. I would decline this option and wait and see if the IRS bills you. If they do, they would likely annualize the shortfall and determine that you were $750 underpaid for the first, second, and third quarters. You would not be underpaid for the fourth quarter because of the estimated payment. The earlier you pay, the less interest you will be charged, and if you pay before December 15, you will be able to avoid the monthly penalty increment that will be charged as of December 15. If the IRS does send you a bill, and you have never owed a penalty before, you can request a first time penalty waiver. The IRS can’t waive the interest, but the interest will be recalculated once the penalty is removed.
