Surface Transportation Extension Act of 2004 - Amends the Internal Revenue Code to authorize through March 31, 2004, expenditures for obligations under the Transportation Equity Act for the 21st Century (TEA-21) from: (1) the Highway Trust Fund; (2) the Mass Transit Account of such Fund; and (3) the Sport Fish Restoration Account and the Boat Safety Account of the Aquatic Resources Trust Fund.
Amends the Internal Revenue Code to establish: (1) a credit against the gasoline and diesel fuel excise tax for an amount equal to the sum of an alcohol fuel mixture credit plus a biodiesel mixture credit; and (2) a general business income tax credit for biodiesel used as fuel equal to the sum of a biodiesel mixture credit and a biodiesel credit.