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4s is lower in energy until there are e- in the 3d orbital (that's why you fill 4s first). Once there are e- in the 3d orbita,l the 4s orbital becomes higher in energy. What helped me understand this is the fact that once there are e- in the 3d orbital they provide "shielding" for the e- in the 4s orbital. Therefore you remove e- from the 4s orbital first when creating an ion.
Until you reach element 19 or 20, the S will generally come before d because it is in lower energy, once the d orbital starts to get filled up, it loses its energy and is less energy than the 4s orbital, for example, scandium would have the configuration of 3d13s2 if it was coming from Argon. Make sense?
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