Statement I : The E^o value for Ce⁴⁺/Ce³⁺ is + 1.74V.
Statement II : Ce is more stable in Ce⁴⁺ state than Ce³⁺ state

Ce⁴⁺+e⁻→Ce³⁺
E∘=1.74 volt
As the E^∘ value here is positive, the reduction of Ce⁴⁺ to Ce³⁺ is feasible.
Though Ce⁴⁺ has a noble gas electronic configuration, but to remove 4 electrons from Ce we need to provide large amount of energy, thus Ce³⁺ is more stable than Ce⁴⁺