Since 4s orbital lies in the fourth shell, and 3d in the third shell, electrons are removed first from 4s. Electrons are filled in the orbitals with lower energy first. As 4s orbitals have a lower energy than 3d orbitals, electrons are first filled in the 4s orbital and then in the 3d orbital.

Why should the 4s subshell be filled before the 3d?

Why should the 4s subshell be filled before the 3d? The 4s subshell has greater spherical symmetry than the 3d subshell. The 4s subshell is farther from the nucleus than the 3d subshell. The 4s subshell is at lower energy than the 3d subshell.

Why 4s subshell is filled prior to 3d button ionization 4s electron are removed first?

According to the aufbau principle the 4s orbital is lower in energy than the 3d orbital hence, it is filled first.