Type N spacetimes whose invariant classifications require the fourth covariant derivative of the Riemann tensor

We describe a pure radiation spacetime of Petrov type N whose invariant classification requires the fourth covariant derivative of the Riemann tensor. The spacetime is the first non-conformally flat example which exhibits this quality and shows that spacetimes with matter and non-zero Weyl tensor can also require high-order derivatives in their invariant classification.