Complete Form W4 so that your employer can withhold the correct federal income tax from your pay. Consider completing a new Form W-4 each year.A W4 form, or "Employee's Withholding Certificate," is an IRS tax document that employees fill out and submit to their employers. As employers, state agencies and institutions of higher education must deduct federal income tax (FIT) from the wages of a state officer or employee. Complete Form W-4 so that your employer can withhold the correct federal income tax from your pay. IRS Form W4 tells your employer how much federal income tax to withhold from your paycheck. You'll be asked to fill one out when you start a new job. The new form has five steps. Employees must fill out step 1 and step 5. The form tells your employer how much federal income tax to withhold from your paycheck.