Why does sulfur bond to 4 oxygens in sulfuric acid?

Sulfuric acid has one sulfur atom bonded with 4 oxygen atoms (not including the Hydrogens). The sulfur atom and each oxygen atom have 2 spaces available in their outer shell. How is it that one sulfur atom will bond with 4 oxygen atoms? I would have thought that sulfur could only bond with up to 2 atoms because it only has 2 spaces unfilled in it's outer shell?