The Massachusetts Employee's Withholding Exemption Certificate, Massachusetts Form M-4, must be completed so that you know how much state income tax to withhold from your new employee's wages. The importance of having each employee file a state withholding certificate as well as a federal Form W-4 cannot be overstated, so make its completion a priority. (The federal form can be substituted for the state form, for state tax purposes, in certain circumstances.)