Understand why 4s orbital has a higher energy than 3d orbital

Why does 4s orbital has a higher energy than 3d orbitals?

RULE: The electron furthest away from the nucleus has the highest energy. <-- why?

1. Since nucleus (proton and neutron) attracts electron, it takes a huge amount of energy to pull electron from the ground state to n=4. This is why when electron moves to n=1 from n=4, it releases a tremendous amount of energy.

2. Since electron in 4s has a higher energy, it is quite unstable (we learn that lowest energy= stable and vice versa). As a result, it takes very little energy to remove electrons from 4s orbital. This is the trend you usually see in transition metal's electron configuration.

3. Since paired electrons repel against each other, 4s orbital  will have a higher energy.