Item 4 is the actual tensioner. It pushes onto the blade or lifter, item 3 ,to stop the cam chain flapping about and maybe jumping the sprockets. In my experience its the tensioner that fails or is between teeth on the mechanism, allowing chain to rattle more than normal until the tensioner takes up another notch. The blade could wear all the plastic covering away and maybe have the steel part rubbing on the cam chain. The tensioner is replaceable externally and easily. The blade isn't. My experience with them is clean the tensioner and if necesary , allow pusher part to come out another notch. Best solution is to replace with a manual tensioner.