I am a bit confused over the rules and requirements for individuals (and not businesses) making income tax-prepayments to the US Treasury. For instance, I've read that you can make quarterly payments, but that if you choose this route, you must make four payments each year (one for each quarter).
This doesn't make much sense to me as someone can sell stock late in Q4, which is not subject to income tax withholding, and then be subject to a potential penalty come April of next year.
Is there a proper procedure that allows one to send the treasury an income tax prepayment at any time? Which forms allow for such a payment?
I realize that one can submit a new W4 form for additional paycheck withholding, but this might not be sufficient as it is limited to the amount of money available on the check, and time left in Q4.