The 4 lug Cobalts (base models) are 4 x 100 mm bolt circle. The five lug Cobalts (SS models) are 5 x 110 mm bolt circle.
If you're going with an adapter spacer where you get new studs with a new bolt circle, a very common one to go with is 5 x 114.3 mm (equal to 5 x 4.5 inch) bolt circle, since the aftermarket wheel world has a lot of choices for that.