Idling Reduction Tax Credit Act of 2004 - Amends the Internal Revenue Code to allow a business tax credit for 50 percent of the cost of a qualifying idling reduction device, up to $3,500. Defines "qualifying idling reduction device" as any device that is: (1) installed on a heavy-duty diesel-powered on-highway vehicle to provide services that would otherwise require the operation of the main drive engine while the vehicle is temporarily parked or stationary; and (2) certified by the Secretary of Energy to reduce long-duration idling. Directs the Secretary to publish standards for certifying such devices.