Chris,
When you work the gear change pedal, do you see an movement of the 4th gear toggle? I am wondering if it is an issue with the gear change linkage that the pedal is not pushing the bus bar down enough for 4th gear strut to release. When I worked on these for the army, we had levers to fit directly to the gearbox (removing the short lever) then we could work the box without the pedal linkage inhibiting movement. Once a box was set up correctly, the pedal linkage could be adjust accordingly. For you, you could find a good fitting length of pipe to slide over the short lever, then drill it to fit a bolt to secure it to the lever.
Richard