回复：Question about TAX underwithholding

正如stp所说，是有特例，如果满足，不用缴纳罚款。比如你上一年度12个月应税额为3000元，今年你的抵扣withhold为3100元，而现在在return上算出你今年的应税额为7100元，即你说的短了4000元。在这种情况下，即已抵扣的3100大于去年全年的应税额，你不用付罚款。具体参看irs.gov的下面描述。

Generally, you should make estimated tax payments if you will owe tax more, than an amount specific by law, after withholding and credits, and the total amount of tax withheld and your credits will be less than the smaller of:

90% of the tax to be shown on your current tax return, or 100% of the tax shown on your prior year's tax return, if your prior year's tax return covered all 12 months of the year. However, if your prior year's adjusted gross income exceeded a certain amount based on your filing status, then you must pay 110% instead of 100% of last year's tax. (Note: the percentages change depending on the tax year. Refer to Chapter 2 of Publication 505, Tax Withholding and Estimated Tax.)