The W-4 tells your employer how much federal income taxes to withhold from your paycheck. As such, it only effects paychecks going forward - it has no effect on paychecks already issued.The amount withheld is simply an eof how much federal income tax you will ultimately owe at year's end. The amount withhold may be more or less than you end up actually owing. If too much is withheld, you are entitled to a refund, if too little, you will owe taxes. You can change your W-4 withholding at any time by submitting an updated W-4 to your employer. You do not have to fill out a new W-4 every year, although your employer may require it.This form is retained by the employer, and is not submitted to the IRS.