833 4 speed no 3 and 4

I think I would start by putting the 1-2 lever In neutraI and disconnecting the
3-4 shift rod from the lever at the trans. See how well the 3-4 lever shifts without the shifter
If all is well install a 1/4" drill bit through the shifter box neutral alignment slot. Reinstall 3-4 shift rod onto lever adjusted to length so it slips right in without any resistance at all. Pull drill bit out and test. Besure rods are not binding somewhere that would keep the lever from clicking into the gear position. If all is still not well I'd open it up and have a look. It is almost always in the shifter linkage. But the fork like mentioned above could be causing your problem.