To find the equation that represents the cost of cookies during the sale, we need to determine the price per cookie after the discount.
Initially, the baker charges $2.00 for 4 cookies, which gives a price per cookie of:
\[ \text{Price per cookie} = \frac{2.00}{4} = 0.50 \]
During the sale, he charges $0.10 less per cookie:
\[ \text{Sale price per cookie} = 0.50 - 0.10 = 0.40 \]
Now, we can relate the total cost \( y \) to the number of cookies \( x \) during the sale:
\[ y = 0.40x \]
So, the correct equation is:
\( y = 0.40x \)