I'm a little confused. Are you able to select 1 - 4 at the gear shift or at the selector shaft in the rear cover?
And even though you've "selected" one of 1 - 4, the tranny is actually in 5th? If you are able to select 1 - 4 while the transaxle is in 5th, that should lock the tranny.
It sounds like 5th was selected but somehow the shift finger escaped the 5 shift fork and is in position to select the other four gears.
I'm assuming you haven't been working on your transaxle - specifically had the end cover off. That being the case, none of this makes any sense to me - which should be taken to mean that I don't know enough about the transaxle to imagine how it could happen. Maybe someone else has an idea about that
If on the other hand, you did open the rear cover of the transaxle, I can easily see how the shift finger was in neutral (or even a gear) and putting the end cover on could have moved the 5 shift fork selecting 5th. On the NG3, and presumably, the 365, it is possible to select 5th and another gear. In fact it's a technique spelled out in the manual. I had to contend with this when putting my NG3 back together. This was much more likely when the transaxle was vertical. Laying it down made it easier to avoid this problem.
I'm thinking that you'll have to take the end cover off, make sure all the shift forks are lined up properly, and put the end cover back on taking care that the shift finger is properly situated and that all six gears can be selected. Actually, if you can choose reverse, any of 1 - 4, and fifth, you should be able to get them all.