A large difference between the third and fourth ionization energies indicates the presence of:

The elimination of the fourth electron for the potential ${\mathrm{ns}}^2{\mathrm{np}}^1$ configuration may come from an inert gas electron configuration. Therefore, the fourth ionisation enthalpy, which results from the ${\mathrm{ns}}^1$ electron configuration, will increase significantly more than the third ionisation enthalpy.