The redshift of NGC 4993 is z = 0.009727.  

  Ned Wright's cosmology calculator indicates that, 
for the standard cosmological model, this corresponds to 


     luminosity distance  dL   =   42.2 Mpc

  
  We can then compute the distance modulus via


      (m - M)  =  5 log10 (dL)  -  5

               =  33.1