  The fourth-order correlations of conserved charges, such as the baryon
number, electric charge, and strangeness, are studied at finite temperature and
nonzero baryon chemical potential in an effective model. It is found that the
fourth-order correlations change rapidly and have three extrema during the
chiral crossover with the increase of the baryon chemical potential. The
absolute values of the extrema approach infinity when the thermodynamical
system moves toward the QCD critical point and the fourth-order correlations
are divergent at the critical point. The contour plots of the fourth-order
correlations in the plane of the temperature and baryon chemical potential are
given. It is noticed that all the fourth-order correlations of conserved
charges, except for $\chi_{13}^{BS}$ and $\chi_{13}^{QS}$, are excellent probes
to explore the QCD critical point in heavy ion collision experiments.
