In this paper, we propose a fourth-order exponential wave integrator
Fourier pseudo-spectral method for the
Klein-Gordon-Schr\”{o}dinger (KGS) equation. The
proposed method is time symmetric and explicit so it is easy to apply by
the fast Fourier transform (FFT). By using the standard energy method
and the mathematical induction, we make a rigorously convergence
analysis and establish error estimates without any CFL condition
restrictions on the grid ratio. The convergence rates of the proposed
scheme are proved to be at the fourth-order in time and spectral-order
in space, respectively, in a generic $H^m$-norm. Extensive
numerical results are reported to confirm our error bounds. Because that
our error estimates are given under the general $H^m$-norm, the
conclusion can easily be extended to two- and three-dimensional problems
without the stability (or CFL) condition under sufficient regular
conditions. The proposed fourth-order method could also find
applications to solve other coupled system such as the
Klein-Gordon-Dirac system.