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The four-current combines charge density and current density into the four-vector $J^{\mu }=(c\rho ,\mathbf{J})$. With $x^{\mu }=(ct,\mathbf{x})$, its four-divergence vanishes by the charge continuity equation: ${\partial }_{\mu }{J}^{\mu }={\partial }_t\rho +\nabla \cdot \mathbf{J}=0$. It is the source in the covariant Maxwell equations.

Take the four-divergence of the Proca equation. Commuting partial derivatives and using that $F^{ab}$ is an antisymmetric second-rank tensor giveConsequently, for $m\ne 0$, ${\partial }_a{A}^a=0$. This is the Lorenz constraint in Proca theory: an equation of motion enforces it, rather than a choice of gauge fixing. It leaves $(\square -m^2)A^a=0$. At $m=0$ the divergence argument supplies no such constraint; the massless gauge symmetry requires a separate treatment.