This is because extracting 4 electrons from an atom requires huge amount of energy. Apart from the noble gas configuration, the increasing charge over the ion further contributes in decreasing its stability. So Ce(IV) only exists just because it has the noble gas configuration at this oxidation state contributing in its stability but the increasing charge contributes in reducing stability so the optimum stability is obtained at oxidation state 3. It requires a really strong oxidising agent to oxidise state 3 to state 4 which must have the electron gain enthalpy greater than the ionisation energy of the Ce(III) which can provide enough energy to knock out electron from Ce(III). But Ce(IV) can be reduced easily to Ce(III) using hydrogen gas.
