JR Buddy Boxes
Being that the 421 is a computer radio and the quattro isn't, I think you might have trouble if the master (the 421) has used any of the computer features like the travel adjust.
I had problems getting an old JR Max to work properly with a 421. I could adjust the servo direction but the travel on the 421 had been adjusted to 50% and the JR Max always wanted to give 100%.
The solution was to change and/or adjust servo arms and control horns so the 421 settings could be set on the 100% mark. The control surface movements were then equal.