The serializer can power 2 DC motors up to 4A each. If you install 4 DC motors (2 motors connected in paralel for each side), the motors need to be up to 2A each.
The Lynxmotion recommends the 4WD1 kit to work with 2x10 Sabertooth controller, that means up to 10A per side, so the motors are up to 5A each!
Actually, looking at the motors (GHM01) that come with the basic kit, they need 1.5A at stall, so they can work with the serializer. However, if other motors are included in the kit (GHM02 and up) you need to look carefull at the motor datasheet to see that the max current does not excede 2A. If it does, you need a different motor controller.