I am wanting to use the 4WD-1 chassis with a slight modification. More specifically I'd like to remove the motors from the two rear wheels and instead have quaderature optical encoders.
You need to add quadrature encoders to the motors.
From the assembly guide, it does not look like the motors will accept quadrature encoders. Only the GHM-04 motors have the extended shafts for the QME-01 encoders. If you have not yet purchased the 4WD-1 rover kit, you might be able to have the stock motors swapped out for the GHM-04 motors. The QME-01 quadrature encoders are not the same as optical encoders - they attach directly to the GHM-04 motors.
My reasoning for leaving the rear wheels unpowered is that if they are not powered I shouldn't have to worry about wheel slippage as much... If the encoder wheels are turning then the robot would have to be moving since the front wheels are doing the pulling.
For optical encoders, this might be an issue, but I don't think it will be for the motors LM sells. I have purchased GHM-04 motors and the QME-01 encoders and am looking forward to having my robot be able to navigate better. I will also be adding a Devantech CMPS-03 digital compass module, so will have a setup very much like you are talking about.