+4 oxidation state of cerium is also known in solutions.
${\text{4f}}^{\text{1}}{\text{5d}}^{\text{0}}{\text{6s}}^{\text{0}}\text{:}{\text{Ce}}^{\text{+3}}$
${\text{4f}}^{\text{0}}{\text{5d}}^{\text{0}}{\text{6s}}^{\text{0}}\text{:}{\text{Ce}}^{\text{+4}}$
The high charge on ${\text{Ce}}^{\text{4+}}$ leads to high hydration, hydrated ${\text{Ce}}^{\text{4+}}$ is hydrolysed giving polymeric species and ${\text{H}}^{+}$. $\text{Ce}\left(\text{+4}\right)$ is stable in strongly acidic solutions and used as oxidising agent in volumetric analysis.