[FrankIII]

Sounds like some of the teeth are broken off one of the 4th gears. Maybe they are worn in the center of the 4th gear, where the shift keys engage.

 

[FrankIII]

The transaxle would have to be removed and dissassembled. I took apart a old Spicer recently (simalar transaxle) to see how it worked. If you can find another transaxle, it might be better than repairing the one you've got. It might be a challenge to reassemble espacailly without a service manual. Seems odd that a gear would fail like that. Did you ever shift without pushing the clutch?