Step 1: Understanding inner transition elements.
Inner transition elements are the elements that are found in the lanthanide and actinide series. These elements have their f-orbitals being filled.
Step 2: Identifying the element.
Among the given options, Cerium (Ce) is an inner transition element, as it belongs to the lanthanide series.
Step 3: Conclusion.
The correct answer is Ce, which corresponds to option (4).