I don't understand why SCNNode's rotation property is a SCNVector4 instead of a SCNVector3. The documentation states that:
The four-component rotation vector specifies the direction of the rotation axis in the first three components and the angle of rotation (in radians) in the fourth. The default rotation is the zero vector, specifying no rotation. Rotation is applied relative to the node’s pivot property.
What's the fourth axis? I tried adding a light (SCNLight) and if I don't specify the value of the 4th float in the vector the light has no effect (the scene is dark). Why? shouldn't a 3-dimensional vector (x,y,z) be enough?