  We investigate the effect of charm quark on the chiral phase transition of
light quarks at finite temperature based on the four-flavor soft-wall
holographic QCD model. In the massless limit, we find that the thermal chiral
phase transition is of the second order in the four-quark flavor system. In the
case with the massive charm quark and the massless light and strange quarks,
the order of the phase transition changes to the first order. This is due to
the quark flavor symmetry breaking which is associated with the violation of
the $U(1)$ axial symmetry. Once the light and strange quarks get massive, the
explicit chiral symmetry breaking becomes eminent, then the crossover phase
transition is realized at the physical quark masses. We also map the order of
the phase transition on a phase diagram in the quark mass plane where the
light- and strange-quark masses are degenerate but differ from the value of the
charm quark mass. This phase diagram is an extension of the conventional
Columbia plot to the four-quark flavor system. The critical exponents related
to the chiral phase transition are also addressed.
