We do not know if making an estimated 4th quarter payment will cover the tax on your RMD since we have no idea whatsoever how much income you have had or how much your RMD will be. When you enter your 1099R for the distribution from the retirement account, the software will reconcile all of your income and all of the tax paid or withheld during the year to determine whether you owe more or get a refund. As far as "when" to take your RMD-- do it before the end of the year. Waiting until December 31 would be a poor choice. Give your financial institution a chance to process the distribution and issue a 1099R to you.