It says HST. Thats for Hydrostatic drive. Not the same as gear binding with a Gear drive transmission. With Kubota, at least mine, you have to pump the brake pedal and clutch pedal alternately at times to get it to shift. Do not force it.
I agree on not running 4wd on hard surface but if I am mowing pasture where extra traction is not needed I don't use 4wd to me its less wear and better fuel mileage.