4x4 on tractors = specific gear ratio between front and rear tires. due to front tires are normally smaller than rear tires. if you get a different size rear tire, then the front tires also need to be adjusted for correct ratio and sized up correctly. dealerships / tire places for agriculture tires should be able to find this data. and give you information of what you need in mixing matching tires up.
beyond changing tire size, i would say there is no way to change speed, beyond the gas pedal / hst pedal. if you want more than that, then you need to get into heavy modifications and build your own tractor.