Amount of substance of a molecule in a solute the same as amount of substance of constituent elements?
You are right thinking that the molar concentration of S will be the same as the molar concentration of SO4, unless there is no other molecule whith S in the sample. 

If there is 1 mole of molecules of sulphate (SO4), there will be exactly 1 mole of atoms of sulfur (S), and 4 moles of atoms of oxygen (O).

On the other hand, though "theoretically" it is correct to say there will be 2 moles of molecules of O2, it is really not correct, because there is absolutely no molecule of O2 in your sample. The amount of oxygen atoms is the same in both cases, but how they are organized is not.

If you want to get the mass concentration of S, say in g/l, you would just multiply the concentration of SO4 in g/l by the mass ratium of sulfur within sulphate, (sulfur mass over sulphate mass, 32/96=0.33333). Since for every sulphate gram there is 1/3 sulfur mass and 2/3 oxygen mass, 1/3 of your sulphate mass is due to sulfur.
Another strategy would be to get the concentration of sulphate, which is exactly the same as that of sulfur (1 mol of atoms of S for every mol of molecules of SO4), and multiply that by the molar mas of the sulfur atom (32).

In brief, the molar ratio between S and SO4 is 1, and the mass ratio is 1/3 grams of sulfur for every gram of sulphate