When the Forms 1099 are received, they show amounts significantly in excess of what was expected based on the previous year's transactions. So it turns out that the estimated tax payments are insufficient. By the time the Forms 1099 arrive, it is too late to adjust the final federal and state estimated tax payments for the calendar year. The Forms 1099 usually arrive in late January or during February, whereas the final estimated tax payments are due on or before January 15. And even increasing the final estimate payment isn't sufficient to avoid estimated tax underpayment penalties because one-fourth of the increased amount of estimated tax that should have been paid would have been due on each of the four estimated tax payment due dates.
What are the possible solutions other than paying the estimated tax underpayment penalties?
One is to make wildly high estimated tax payments on the assumption that the year's brokerage income will increase significantly, to the order of doubling or tripling. I knew someone who did this, mostly because of an unwillingness to have an amount due when filing and an aversion to incurring the estimated tax underpayment penalty.
Another is to ask the brokerage to provide tax relevant information throughout the year. Good luck with that.
Yet another is to ask the brokerage to withhold federal and state taxes on amounts falling into those categories even if not distributed to the client. Though some brokerages may be willing to do something along those lines, many don't or won't or can't.
Fun. Not.