As I said originally, there are two formulas that the employer must choose from to withhold taxes on bonus checks. One formula is a flat 25%— if the employer uses this, they will probably not take an extra amount as specified on the W-4. The other formula is to treat the check as if it was a regular weekly paycheck and apply the W-4, which includes a baseline amount based on your salary and filing status plus any extra amounts that you request.
