In the special theory of relativity four-force is a four-vector that replaces the classical force; the four-force of the four-vector a is defined as the change in four-momentum over the particle's own time:

<math>{dP_a \over d\tau}.<math>

In four-vectors formalism the electromagnetic field is described by a second-order tensor (which is written as a 4 on 4 matrix).

See also: four-vector, four-velocity, four-acceleration, four-momentum.