1. Let's clarify what "part 4" refers to in the context of the problem you are working on. 2. If part 4 is a step or a sub-question in a multi-step problem, please provide the exact statement or the problem so I can explain it clearly. 3. Without the specific problem or context, I cannot provide a precise explanation. 4. Please share the details of part 4 or the problem it belongs to, and I will explain it step-by-step with formulas and examples if needed.