The quantization of the electromagnetic field between moving boundaries in four-dimensional spacetime is studied. We consider the particular case of a field confined between two infinite conducting plates moving with constant relative velocity. It is shown that this motion produces squeezed states and leads to the production of particles at the expense of the Casimir energy. Analytic formulas for the Bogoliubov and the squeezing coefficients are obtained. The energy momentum tensor and its spectrum are evaluated from the field correlations.