Was this ever resolved? I'm at the same place. Can't add that 4th up20 robot. It's advertised all over the place that XRC can control up to 4 robots. I bought the cables for a QR4C from Motoman. I downloaded the QR4C manual from Motoman. I've got the schematic for how to install the cables into 4 robots from Motoman. But don't have an answer on why I can't or how to add that 4th robot.
Let me know!