The default treatment of income as being received uniformly throughout the year is the problem. Assuming that your balance due will be more than $1,000 without a Q4 estimated tax payment, to avoid underpayment penalties for Q1 through Q3 you'll almost certainly need annualize income by filing Schedule AI of Form 2210 unless you sufficiently over-withheld throughout the year. Unlike withholding, a particular estimated tax payment is applied when paid rather than being being applied equally throughout the year.
If you have variety of income and deductions throughout the year, figuring out what income and what deductions occurred in which quarter can take a lot of effort. When I've prepared Schedule AI, I've found it to take as much time as the entire remainder of my tax return.
