28.1.4—Why transition elements have variable

The 3d and 4s subshell energies are close enough that electrons from both can be removed or involved in bonding. This supports multiple stable oxidation states.

The 4s electrons are removed before 3d when forming ions, but once the ion forms the relative energies and ligand environment influence stability. Use actual configurations for the species named.

Fe can form Fe²⁺ ([Ar]3d⁶) and Fe³⁺ ([Ar]3d⁵); both are common because the energy difference is not prohibitive.

Do not say 4s always fills and empties independently of 3d or assume every oxidation state is equally stable.