There is a difference between paying estimated tax vs. having tax withheld.
If you have tax withheld, you can have it withheld in December and that amount is added together with all your other withholdings. So, other than having the withholding occur sometime during the tax year, time is not of the essence for withholding.
If you pay estimated tax, the IRS (and, I think, all states) compares the time (usually by quarter) you paid it compared with the time (again by quarter) that your income occurred. If you are even a quarter late, additional charges apply because time is of the essence for estimated tax.
E.g., see
http://www.irs.gov/publications/p505/ch04.htmlIf these capital gains and your current W-4 mean you don't meet any of the "safe harbor" provisions for avoiding underpayment penalties, increasing withholding in December by enough to avoid the penalties is the most cash flow efficient way to do so. Up to you to decide how closely you want to play that game, including allowing for payroll department glitches....
Anybody know why brokerages don't provide the option to do withholding on request?