What prevents the expression of a recessive allele

Usually the presence of a dominant version of allele masks the expression of a recessive allele. A recessive allele is only phenotype is only expressed when 2 recessive alleles combine. 

Dominant alleles prevent the expression of a recessive allele, unless it is a genotype of two recessive alleles.

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.