If my memory serves me right, a quatre quarts cake is French for a Pound Cake (which is 4 ingredients of 1/4lb (quart de livre) each: egg, flour sugar and butter). (1/4lb = 150g approx)
You are using the metric system. This question is relevant then: cornstarch is sometimes called corn flour outside of North America. Is your corn flour a white or yellow powder?
if white (cornstarch), your cake crumb will be more moist with the added sourcream. It will not be that much different. If yellow (corn flour), your cake will have a coarser crumb, still moister and will have a deeper yellow colour than the original recipe.
If you posted your recipe, it would help us to give you a better answer.