Electronically file this form on MyTax Illinois at mytax.illinois. Please complete a paper or electronic PDF Form W-4 to modify state tax withholding.Reference the table below to see a list of state tax forms. An Estate Tax Payment Form must be completed when submitting payments: Estate Tax Payment Form (PDF fill-in form). Illinois' flat income tax rate means that every resident, regardless of income level, pays the same individual income tax rate of 4.95 percent. All Illinois Courts must accept these forms. This Easy Form helps you provide financial information and documents to the other party and the court. The State of Illinois has easy, fillable forms you may use for certain matters. Click here to view or use the Standardized State Forms. INCOME TAX — LLCs may or may not be subject to all state income tax laws, including withholding taxes.