The four critical skills of a product manager are market research, product development, project management, and product marketing. Each of these skills are important in their own right, and a product manager must be proficient in all four in order to be successful.
Market research is important in order to understand the needs of the target market, and to determine what features the product should have in order to be successful.
Product development is important in order to create a product that meets the needs of the target market, and that can be manufactured at a reasonable cost.
Project management is important in order to ensure that the product is developed on schedule and within budget.
Product marketing is important in order to create demand for the product, and to ensure that the product is positioned correctly in the market.