Final answer:
The Herfindahl-Hirschman Index (HHI) measures the concentration of a market. In this case, with four firms each having a 25% market share, the HHI would be 0.25.
Step-by-step explanation:
The Herfindahl-Hirschman Index (HHI) measures the concentration of a market. It is calculated by summing the squares of the market shares of all firms in the market. In this case, since each of the four firms has a 25% market share, the HHI for this market would be 4 * (0.25)^2 = 0.25.