Explanation:

- We know 20 people work 8 hours a day for 15 days to complete the task.

- Total work can be seen as: 20 people × 8 hours/day × 15 days = 2400 person-hours.

- We want to complete the same work in 4 days with 60 people.

- Let the required hours per day be $x$.

- The equation is: 60 people × $x$ hours/day × 4 days = 2400 person-hours.

- Solving for $x$: 240x = 2400; $x$ = 10 hours/day.

- Therefore, for 60 people to complete in 4 days, they need to work 10 hours a day.

- Correct Answer: Option 4, 10 hours/day.