Here is an issue you may face with the 4:1 that are a 90:1 ratio and the 3:1 that are 39:1 to get the same speed as the 3 you need to be close to 2 1/2 times the on the motor speed. Going to the 3:1 boxes and a motor that makes more low end torque but still stays under the amp rating may give you both of your goals. I have a ZTZ99A and the only boxes that will fit are the 3 to 1 mid lows. The difference between the V2's and the stock HL gear boxes is night and day. They had a lower ratio 39:1 verses 30:1 and the are still faster and climb better than stock and it has metal tracks drive gears and idlers. Each tank will be different based on the mix of parts and the running goal so it may take a little experimentation.
Jimmy