Why is the +3 oxidation state of Cerium more stable than +4?
Are you sure $\ce {Ce}$ (III) is really considered more stable than $\ce {Ce}$ (IV)? Since the most common cerium based compound is its dioxide so $\ce {Ce}$ (IV) is actually more stable.

In addition to that, $\ce{Ce}$ has 4 electrons in its outer orbital, thus leaving only one unpaired electron on the $\ce{6s}$ orbital doesn't sound really stable to me.