Here's why:

* Electron Configuration: The general electron configuration for transition metals in period 4 is [Ar] 3d¹-¹⁰ 4s¹. This means they have a filled Argon core ([Ar]) and are filling their 3d orbitals.

* Orbital Filling Order: While the 4s orbital is filled before the 3d orbital, the 3d orbitals are the ones that are being filled across the transition metal series.

Let me know if you'd like more information about the electron configuration of specific transition metals!