What happens if a calendar year taxpayer files for an extension by April $15$th but does not pay their tax due until October $15$th$?$

No penalties or interest will be charged since the extension covers both filing and payment.

The taxpayer will face penalties and interest because the extension only applies to filing, not to payment.

The taxpayer will receive an additional $6-$month extension for payment by filing Form $4868.$

The tax due is automatically deferred until October $15$th with no need to file any forms.