FourLaplacian[exp, p, q] is \frac{\partial}{\partial p_{\mu }} \frac{\partial}{\partial q_{\mu }} applied to exp.
FourLaplacian[exp, p, q]
exp
Overview, FourDivergence, ThreeDivergence.
SP[q, q] FourLaplacian[%, q, q]
\overline{q}^2
2 D