You're right, trying to plan your taxes when you have capital gains is a challenge.
The easiest way for you to play it safe is if:
- You pay 100% of your 2023 taxes, (110% for higher incomes).
- If your previous year's adjusted gross income was more than $150,000 you will have to pay in 110% of your previous year's taxes to satisfy the "safe-harbor" requirement.
I don't know what you mean by "Is it possible to pay more tax in January for the prior year."?
- But yes, if you think there will be a shortfall in your tax payment
- You can make a larger single payment by April 15th 2024'
- Or you can break it into 4 equal payments.
- You can have TurboTax prepare payment vouchers.
- It sounds like you tried to prepare your W-4
- You can use the Capital Gains from 2023 for your estimate.
- If you had a loss, I would not include it.
- Or you can setup an account and pay directly to the IRS Direct Pay With Bank Account
For additional information about IRS penalties please refer to: Underpayment of Estimated Tax by Individuals Penalty