California Employment Law
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The W-4 is drafted to calculate the amount of withholding required under circumstances where a taxpayer has the least number of deductions available. If you are in a situation where you have few if any deductions, other than the standard deduction, then you are likely to owe the amount of taxes which you are remitting based upon the W-4 calculation.
However, an extra $400 per month does seem excessive, so it may be that you are miscalculating. Since I don't know your financial circumstances, I can't conduct the calculations for you. However, you can use an online calculator with which to check your work. See this link.
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