A four-vector V is defined as:
where the upper index denotes it to be contravariant. The covariant form can be obtained using the metric tensor g:
Often (but not always) the metric is diagonal, meaning the only difference between covariant and contravariant four-vector components are signs, though the signs depend on which metric is used.
Read more about Four-vector: Four-position, Dynamics, Quantum Theory, Physics of Four-vectors