$\underset{\_}{\underset{\_}{J}}=\left(\text{j}\rho c_0;\overrightarrow{J}\right)$

where $\rho$ is volumic electric charge, $\overrightarrow{J}$ is electric current density, $\text{j}$ is the imaginary unit, and $c_0$ is speed of light in vacuum

Note 1 to entry: The coherent SI unit of four-current is ampere per metre squared, $\text{A}\cdot {\text{m}}^{-2}$.