Explanation:

- Differentiated Product: In monopolistic competition, firms offer products that are similar but not identical. Each firm has some power to set prices due to product differentiation.

- Selling Cost: Firms engage in advertising and other selling efforts to differentiate their products and attract customers. This is a key characteristic of monopolistic competition.

- Imperfect Knowledge of the Market: In monopolistic competition, consumers and producers do not have perfect information about the market. This leads to varying perceptions of product differences.

- All the above: Monopolistic competition displays all the mentioned features, making option 4 correct.