If it is actually shifting there should be one gear that it doesn't do it because of the way that the solenoids are energized, what I would do now is with the engine off and the key on, put the hydro handle ahead and shift to 4th gear, using 4 small pieces of metal stick one on each shift solenoid on the wheel motors as they all are magnetised in 4th gear, then start pulling and moving wiring harnesses to the wheel motors to see if you can find a wire that will drop the piece of metal.