docs: def plot4(self, num):
          """
             Plots the abundances of H-1, He-4, C-12 and O-16.
          """
          self.plot_prof_1(num,'H-1',0.,5.,-5,0.)
          self.plot_prof_1(num,'He-4',0.,5.,-5,0.)
          self.plot_prof_1(num,'C-12',0.,5.,-5,0.)
          self.plot_prof_1(num,'O-16',0.,5.,-5,0.)
          pyl.legend(loc=3)