Private Bonds for Modern Roads Act of 2004 - Amends the Internal Revenue Code to include as a tax-exempt facility bond any bond that is part of an issue 95 percent or more of the net proceeds of which are to be used to provide surface transportation facilities. Defines "surface transportation facilities" to mean certain highway facilities and surface freight transfer facilities. Limits the aggregate face amount of a bond issue that may be treated as tax-exempt under this Act. Exempts such bonds from the State cap on volume for private activity bonds.