The 4th quarter payment is due by Jan 15, 2021.
But you may not need to pay it. There is no penalty due if either of these are true:
- 1. You expect to owe less than $1,000 in tax for the current tax year, after subtracting your withholding and credits.
- 2. You expect your withholding and credits to be more than the smaller of: 90% of the tax to be shown on your current year’s tax return, or 100% of the tax shown on your prior year’s tax return (110% if your previous year's adjusted gross income was more than $150,000). Paying 100% of the prior year tax liability is the usual “safe haven”.
If it is necessary to make a 4th quarter payment, then you will want to prepare form 2210 with your return (rather than letting the IRS do it), utilizing the "annualized method" to account for the 4th quarter windfall. Once you initiate form 2210, as described by VolvoGirl, the TurboTax interview will ask if you want to use the annualized method. You'll have to do some calculations.
