A W-4 is a form that an employee fills out so that his employer knows how much federal income tax to withhold from his pay. The Internal Revenue Service, or IRS, recommends that a person fill out a new W-4 form each year and when a person's situation changes.
Reasons why a person must fill out a new W-4 form, also called an Employee's Withholding Allowance Certificate, include getting married or divorced, adding a dependent or getting a new or second job. Since a W-4 form does not follow a person from job to job, a new form must be filled out with each job change. A person can also designate a withholding exemption under certain circumstances.