If you owe money to the IRS, you will need to send in your payment by April 15th in order to avoid interest or late payment penalties. However, even if you cannot pay the IRS the amount owed, you should either file your tax return or file an extension by April 15th in order to avoid late filing penalties. Late filing penalties are a lot larger than interest or late payment penalties. A Form 4868 extension, gives you an extra 6 months to file your tax return, but it does not give you an extra 6 months to pay any taxes owed to the IRS.