回复：Question about TAX underwithholding

if u owe IRS less than 1K, there will be no penalty, if 4K like u described, u will have to pay a interest for that amount, which is around 11% (3% above prime?). Don’t worry, just pay that 4K first, and then IRS will send u a bill. They calculate the interest rate base on which quarter u owe what amount. So 11% is the worst case, normally smaller than that.