By default, income and tax withholding, but not estimated tax payments, are treated as being received and paid evenly throughout the year. This means that there would be an underpayment of taxes for the earlier tax quarters if a large amount of income received late in the year for which an estimated tax payment is made. Eliminating the penalty for the earlier quarters requires annualizing income on Schedule AI of Form 2210 to cause income to be treated as received unevenly throughout the year. Schedule AI is where you would indicate that all of the income from the Roth conversion occurred in Q4.
