Form W4is a document by the IRS that employees fill in order to enable their employer to deduct the right amount of federal income tax on their salary. It contains such personal information as filing status, dependents, and other income or deductions. To complete it correctly can assist in avoiding under-withholding (it will result in the tax bill) and over-withholding (it will lead to a larger refund and smaller paychecks). Whenever there are changes in life, employees are expected to change their W-4 such as marriage, divorce, new dependents or when they get a second job. Filling in the W-4 correctly would help you to make sure that your tax withholding would match your actual tax bill during the year.