Hey guys. Anyone know any pros and cons of the 4 speed vs the 5 speed gearboxes, also what they both rev at going 100kph?

The 5 speeds I have seen had a 4.833 diff ratio and 0,82 fourth gear.(3.96 final drive)
This is taller, but very close to the stock 2lt bus box over all ratio of 4.57 and 0.88.(4.036).
There were some late 4 speeds that had the 4.57 dff and 0.82 4th gear, but seem less common than the 5 speeds in Australia.

It should be noted that the 4 speeds came with selector shaft in the :
nose,
or side of nose cone,
or side of gear casing.

The five speed only came in side shift,
which the selector shaft comes from the side of the main casing,
so will not fit easily in earlier buses.

The 5 speed transporter box is essentially a four speed with a low first gear.
4th/5th gear CAN be improved at a rebuild.

Thanks for that. Ideally it would be good to put a tall final gear in to allow it to rev lower on the highway. Do you know how much it would cost?

Albins sell the cogs.
Last one I got was about $405 to buy.
You favourite local trans builder can do a quote for you.