The idea of a four speed Motom came from studying the vijf speed project I'm working on.
A stock Motom has an extra shifting possibility for starting the engine with the petals.
You can use that shift point for the fourth gear.
Possible to use the gearshift mechanism.
Enough room for the last gear, it stays inside the normal cranck case.
Should be doable.
Drill a hole for the ball, and seal it off with a spring lock.
Enlarge the shaft with 8mm, ore make a new one.