The correct answer is option 3, which states that economic models can only be expressed through equations. This statement is not correct because economic models can be expressed in various forms, including equations, graphs, diagrams, and narratives. Economic models are simplifications of complex real-world processes that aim to understand and explain economic phenomena.

Option 1 states that economic models involve the simplification of complex processes, which is correct. Economic models take complex economic systems and simplify them into manageable components to analyze and understand their behavior.

Option 2 states that economic models represent the whole or a part of a theory, which is also correct. Economic models are often built based on underlying economic theories and concepts, and they represent these theories by capturing the relationships between different economic variables.

Option 4 states that economic models help in gaining insight into cause and effect, which is true. Economic models allow economists to analyze the relationships between different variables and understand how changes in one variable can impact others, providing insights into cause and effect relationships.

Therefore, the correct answer is option 3, and the statement that economic models can only be expressed through equations is incorrect.