A W-4 is filled out by employees to provide their employer with their tax ID number (usually SSN), marital status, number of allowances and dependents, and how much tax to withhold with each paycheck. The W-4 is filled out when an employee is first hired or if any changes must be made to filing status or withholding. The W-2 is filled out by employers at the end of the tax year and sent to employees to input on their tax returns.