When building Period 4 atoms, the [...]\text{[...]}[...] sub-shell fills before the 3d3\text{d}3d sub-shell because it is at a lower energy level.
When building Period 4 atoms, the 4s\boxed{4\text{s}}4s sub-shell fills before the 3d3\text{d}3d sub-shell because it is at a lower energy level.
