I’d assume only a single set “DFRobot Wheel Encoders for DFRobot 3PA and 4WD Rovers (2pk)” are required for the 4WD Rover? Correct?
If just a single set should they go on the front or rear wheels? Does it matter?
You can choose to either mount encoders to the front wheels or to the rear wheels.
Mounting an encoder to each wheel is best to account for slipping.