Notwithstanding the provisions of section 358(a)2 of this title, the rate of contribution required to be paid under the Railroad Unemployment Insurance Act [ 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 , and before .145 U.S.C. 351December 31, 19611961-12-31January 1, 19641964-01-01