Notwithstanding the provisions of section 358 (a)2 ^[1] of this title, the rate of contribution required to be paid under the Railroad Unemployment Insurance Act [45 U.S.C. 351 et seq.] by every employer as defined in such Act shall be 4 per centum with respect to compensation as defined in such Act, paid after December 31, 1961, and before January 1, 1964.