Answer
In order from increasing energy, the orbitals are: 1s $\lt$ 2s $\lt$ 2p $\lt$ 3s $\lt$ 3p $\lt$ 4s $\lt$ 3d $\lt$ 4p $\lt$ 5s. The 4s orbitals will be filled before the 3d orbitals, just as the 5s orbitals fill before the 4d orbitals. They are lower in energy because of greater penetration of the 4s and 5s orbitals.
Work Step by Step
In order from increasing energy, the orbitals are: 1s $\lt$ 2s $\lt$ 2p $\lt$ 3s $\lt$ 3p $\lt$ 4s $\lt$ 3d $\lt$ 4p $\lt$ 5s. The 4s orbitals will be filled before the 3d orbitals, just as the 5s orbitals fill before the 4d orbitals. They are lower in energy because of greater penetration of the 4s and 5s orbitals.
