In diagram B, the position of the individual supplier of fried chicken pieces, Funky Chicken, is represented. Since Funky Chicken is a price taker, it has to take the price of R4 as given and sell its fried chicken pieces at R4. By extending the market price of R4 to diagram B as a horizontal line, the demand curve facing the individual firms is derived. This demand curve is perfectly elastic and indicates that Funky Chicken can sell any quantity at a price of R4. Remember that Funky Chicken has a small market share since there are many sellers in the market.
Why can it not sell chicken pieces for R5? It is selling the same product as other suppliers, and since buyers know that they can obtain a piece of fried chicken for R4, nobody would be prepared to buy it for R5. Why not sell a piece of fried chicken for R3? Because Funky Chicken knows that it can sell all its fried chicken pieces for R4, and because it wishes to maximise its profits, it would be irrational for the business to sell it for R3 if it can obtain R4 for a piece of fried chicken.