Use the following pages to complete your Illinois State and Federal income tax withholding, or W-4 information. The Illinois Form IL-W4, Employee's Illinois Withholding Allowance Certificate, must be completed so that you know how much state income tax to withhold.Caution: If you don't return Form W-9 to the requester with a TIN, you might be subject to backup withholding. See What is backup withholding, later. Have each of your employees fill out a Form ILW4. Register with the state using the MyTax Illinois portal.