No person shall be compellable to serve on the grand or trial jury of the superior court or on any jury in other courts for more than four weeks in any year. No person shall be allowed to serve on the trial jury of the superior court or as tales juror in any criminal case or on any jury in other courts for more than four weeks in any one year unless he is actually engaged in the trial of a case when the four weeks expire, in which case he shall be discharged as soon as the case is decided.