Typically when a manual trans jumps out of gear its because the engagement teeth on the speed gear are worn.  Those teeth are designed to mesh closely to the sliding sleeve, and hold the sleeve on the gear.  When they get worn the sleeve can slip back off, often when you first lift your foot off the throttle to decelerate.  I do not believe this issue would be caused by a worn out shifter, though it may well have caused the damage to 4th gear. 

Unfortunately 4th gear in a Toploader is also the input shaft.  The good news is that you can remove the input shaft without having to completely disassemble the trans (though, if the trans is already out and it has this problem so close to a "rebuild" I would go through the whole thing to make sure everything was right). 

rtmjr13@optonline.net wrote:

Thanks  Senior 24-7

Is it Senior, or Señor?
 

Yeah, what usually happens is after enough bad shifts the tips get rounded off and wear starts to migrate down the body of the tooth.  My metric is whether on not there's a sufficient bur raised on the teeth to catch my fingernail.  If there is I know that if its not a problem now its not long for this world and would replace the gear in any of my personal vehicles and highly suggest it to any of my customers.  Some guys will just polish the bur off and throw it back into service, but the problem is that may smooth out shifting temporarily, but they can't put the lost metal back on the tooth, so its in essence using a compromised part.  A couple more bad shifts, or even just running down the road too long with your hand resting on the shifter can cause even more wear and then it starts grinding going into gear, or jumping out of gear.