Why do PES graphs list 4s after 3d?
For a given atomic kernel, the orbitals energies are not fixed like a set of shelves. Their energy depends on filling themselves and also on filling the other orbitals by electrons, because of kernel shielding and mutual repulsion.
The mutual energy is $4\mathrm{s} \lt 3\mathrm{d}$ when empty. But as kernel charge gets increased and $\mathrm{4s}$ and then $\mathrm{3d}$ are being filled, their energy order gets switched.
The typical case is the 3d energy drop $\ce{K -&gt; Ca -&gt; Sc}$, where the order is:
$$\mathrm{K: 4s \lt 4p \lt 3d}$$
$$\mathrm{Ca: 4s \lt 3d \lt 4p}$$
$$\mathrm{Sc: 3d \lt 4s \lt 4p}$$
well described on  charts in SE Chemistry Q what-is-d-orbital-collapse, which are related to the effective kernel charge acting upon electrons on various orbitals.