Because the 2020 Form 1099-R reports tax withholding, the Form 1099-R must be entered into both 2019 TurboTax and 2020 TurboTax. In 2019 TurboTax you can enter the box 4 amount or not; 2019 TurboTax knows to ignore box 4 of a 2020 Form 1099-R. Just be sure that, when TurboTax asks in regard to the code P, you indicate that it's a 2020 Form 1099-R.
Because there are no taxable gains present in box 2a, the only effects on your 2019 tax return will be the inclusion of the box 1 amount on 2019 Form 1040 line 4a and the inclusion of your explanation statement for the return of contribution. When reporting the same Form 1099-R again on your 2020 tax return by entering it into 2020 TurboTax, the only effect on your 2020 tax return will be the inclusion of the tax withholding on the tax-withholding line of 2020 Form 1040.
