[Jacobian] When I deleted the original 4-th bone, the new end factor could not reach the target

[shrinktofit]

Helps wanted! So excited found this repo, I'm researching on inverse kinematic solutions.

My change:

The result:

[tomcatmew]

Sorry for the late reply.
This bone4 actually stands for "EndEffector" which represents the offset position of the End Effector relative to the last bone.
This bone4 is NOT drawn because it is just a representation of the End Effector of the joints. You can see on the mesh coordinate table of the rendered box. The End Effector has all 0s because we don't need to render it.

If you remove the bone4, which means you remove the EndEffector,
The bone3, (which is the blue bone), uses the End effector to compute the rotation. Without the End Effector, the blue bone doesn't know where to rotate.

The way how I write the code and joints system is quite messy. I should find a more readable way to describe them. I will try to improve the code later to make it easier to understand.