When i was hill climbing we ran 4th gear it was one to one .
Otherwise you would slpit the case open (British Stuff) .
As Ted said you need to split the cases . Spacers would keep you from false neutrals . The weight savings might not be worth the work .
You might be able to remove the attending gears and modify the shift mechanisim for only neutral and 4th or make a gate for the shifting . To avoid the false neutral .