The elements in which the additional electrons enters $(n-2) f$ orbitals are called inner transition elements. $4 f$-block elements are also called lanthanides or rare earths so lanthanides are 14 elements in the sixth period (atomic no. $=58$ to 71 ) that are filling $4 f$-sublevel.
Related Theory
Lanthanoid contraction effect results from poor shielding of nuclear charge (nuclear attractive force on electrons) by 4 felectrons so effective nuclear charge increases; the $6 s$ electrons are drawn towards the nucleus, thus resulting in a smaller atomic radius.