$\underset{\_}{\underset{\_}{p}}=\left(\text{j}E/c_0;\gamma \overrightarrow{p}\right)$

where $E$ is sum of its rest energy ($m_0{c}_0^2$) and its kinetic energy

Note 1 to entry: $\underset{\_}{\underset{\_}{p}}=m_0\underset{\_}{\underset{\_}{u}}$ where $m_0$ is the rest mass and $\underset{\_}{\underset{\_}{u}}$ is four-velocity

Note 2 to entry: The coherent SI unit of four-momentum is kilogram metre per second, $\text{kg}\cdot \text{m}\cdot {\text{s}}^{-1}$.