  We have studied the influence of alloying with a fourth element on the
temperature of ferromagnetic ordering $T_C$ in Ni-Mn-Ga Heusler alloys. It is
found that $T_C$ increases or decreases, depending on the substitution. The
increase of $T_C$ is observed when Ni is substituted by either Fe or Co. On the
contrary, the substitution of Mn for V or Ga for In strongly reduces $T_C$.
