Occupy 4s before 3s but remove 4s before 3s orbital

As the shells of electrons get further and further from the nucleus, successive shells become closer in energy. Thus the difference in energy between the second and third shells is less than that between the first and second.

By the time the fourth shell is reached, an overlap between the highest energy orbitals(3d) in the third shell and the lowest energy orbitals(4s) in the fourth shell. Hence electrons fill in the more stable 4s orbitals first, before 3d orbitals are filled. As 4s orbital is at a lower energy level than 3d.

Once the 3d level is occupied by electrons, these repel the 4s electron even further from the nucleus. The 4s electrons are pushed to a higher energy level higher than the 3d level now occupied. Hence, transition metals lose electrons from the 4s level before the 3d level.