(4) For purposes of this subsection, and notwithstanding the provisions of paragraph (2) of this subsection and any provision contained in the National Bureau of Standards Handbook or any other national standard that may be adopted by law or regulation, the gallon equivalent of compressed natural gas shall be not less than 110,000 British thermal units. As used in this paragraph, the term "compressed natural gas" means a mixture of hydrocarbon gases and vapors, consisting principally of methane in gaseous form, that has been compressed for use as a motor fuel.