Following hierarchy of Nodes:
rootNode -> secondNode-> thirdNode-> fourthNode
I had to move fourthNode along the y-axis of thirdNode. Worked.
But it doesn't work correct anymore if I do a KeyNodeRotateLeftAction or KeyNodeRotateRightAction on secondNode and then try this mentioned moving of fourthNode. The translation along y-axis is done correctly, but from now on the x- and z-translation has an demonic influence: fourthNode now races through the scene, because out has now x- and z-values not equal to zero.
// y is wished translation along y-axis
Vector3f in = new Vector3f(thirdNode.getLocalTranslation().x, y, thirdNode.getLocalTranslation().z);
Vector3f out = new Vector3f();