I've recently built the AR4 version of the arm. So far it's working reasonably well, but I do have a few issues.
One such issue I'm currently facing is that there is a large amount of free play in the J4 joint. Specifically, the joint rotates freely about 20 degrees in either direction when the motor is energized.
This isn't exactly the same as traditional backlash because the shaft still rotates accurately as long as there is no external force acting on it. Howerver, if one of the cables are tug on the joint, it can lead to inaccurate movement. For example, if the motor is commanded to move 25 degrees, it may end up only moving 5 degrees if one of the cables resists movement a little bit.