Today I had a look on how the MT75 is build up and 4th gear is direct link between the input shaft and main shaft, so not using any gears. So I guess that means it shouldn't break either and 4th gear should be quiet on a semi-straight cut gearbox, because no gears are used in that situation.

I also looked at the Oppliger semi-straight cut gear set and it looks like it's doesn't include the 5th gear cog for the main shaft. So if it's meant to use the original 5th gear that should be quiet too, but in that situation it's still using the new gears on the input shaft and the layshaft which might be noisy.