First of all, welcome to the Thor Community! 🙂
The DIY bearing of the 4th articulation is bit tricky to print because of 3D printing tolerances. The current piece fits with my printer tolerances, but yours could be different, or maybe we are using different plastic balls. The solution is to modify the bearing pieces, increasing the torus radius. As it could took several attempts I would recomend you to design a simple bearing with two cylinders and tune the radius of the torus, and when you got the good one, modify the original radius and print the art4 bearing pieces 😉
If you need some help with the process do not hesitate to ask!