Which orbital is filled immediately before the 4f subshell?

$\text{ One }{e}^{-}\text{goes to }5d\text{ sub-shell before the }4f\text{ sub-shell }$

$\text{ The electronic configuration of }\mathrm{La}(Z=57)\Rightarrow [\mathrm{Xe}{]}^{54}6s^{2}5d^{1}4f^0$

$\mathrm{Ce}(Z=58)\Rightarrow [\mathrm{Xe}{]}^{54}6s^{2}5d^{1}4f^1$