Sulfur trioxide - vacant d-orbitals
I feel you are confused on atomic orbital and molecular orbital.

In the excited state of a single sulfur atom, electron do fill 4s first. You are right here. However, it is a different story in molecules. 

In sulfur trioxide the bonding electrons are filling the molecular orbital, not the atomic orbital such as 4s or 3d. It is forbidden that both 3s and 4s participate any hybridization or formation of molecular orbital. So no matter how low is the energy of 4s, because 3s is already participating, 4s can not be part of the bonding.