I've been reading through both the AR3 and AR4 manuals. Other than the J1 Base Enclosure and the alternate RJ connector holes, no 2 pin avionics connectors I don't see any other hardware modifications. (belt verses chain is possible on both). And of course the motors have the built in encoders.

Are there any other hardware changes?

I remember reading that the chance from the Teensy 3.5 to Teensy 4.1 was to move much of the motion code into the Teensy out of the PC. But why the change from the MEGA 2560? Does that mean the AR4 software won't work with the MEGA 2560?

Is there an overall detailed differences list between AR3 and AR4?