Cerium (Z=58) has electronic configuration `[Xe]4f^(1)5d^(1)6s^(2)`. By losing three electrons, `Ce^(3+)` ion has the configuration `[Xe]4f^(1)`. It has quite obviously an urge to get rid of `4f^(1)` electron as well to achieve a noble gas configuration. Therefore, it also shows +4 oxidation stae in its compounds.
