4 spd. pops out of gear.

Common issue.

Adjust the linkage per the directions in the FSM for a baseline.

If it still happens, move the adjustable swivel in or out one turn, try it again.

I have not had one yet that took more than two turns.

There was no amount of adjustment that would keep my duster in 2nd gear on hard deceleration. It always popped out if I engine braked it going downhill at any significant speed, and I played around with the adjustment quite a bit. While the adjustment is a factor, the overall clearances on the shafts is ultimately the source.