You must complete Form IL-W-4, Employee's and other Payee's Illinois Withholding Allowance Certificate and Instructions, on or before the date you start work. Use the following pages to complete your Illinois State and Federal income tax withholding, or W-4 information.Information about Form W4, Employee's Withholding Certificate, including recent updates, related forms and instructions on how to file. The w-4 has five steps we'll go through each step briefly then show an example of a young person completing his first w-4. The form is divided into 5 steps. The IL-W-4 form is fairly straightforward. The employee must enter their name, social security number, street address, city, state, and zip code. If you are unsure how to fill out the withholding form, you can click the View. Blank Form button for a PDF copy of the form or contact. Company field is set to.