The market for a product has the following demand and supply equations: Qd = 500−50P and Qs = 150 + 25P, where Qd and Qs represent the quantity demanded and supplied respectively, and P represents the price of the product in dollars. If the market price is set at $4, what is the excess demand (shortage) in this market?
a. 50 units
b. 100 units
c. 150 units
d. 200 units