Jumping out of fourth is often related to bearing wear either the large ball,or the sleeve bushing,or both .It may also be caused by worn selecter forks,or a weak spring in the detent.If it jumps in and out of gear,expect widespread damage from gear chips .The engagement dog teeth will also be worn, possibly requiring gear replacement..The main problem these bikes had was misalignment of the main bearings,apparently caused by a lack of interest at the works.Regards John.