Explanation:

Let's look at the passage carefully:

- The passage is about preparing to fall asleep at night.

- The first sentence advises turning off electronic devices at least 30 minutes before bed.

- It suggests relaxing the eyes by avoiding lit screens.

- It offers alternatives like reading or writing.

- In blank 4, we need a preposition to connect "write" and "your to-do list."

Now, let's examine the options for blank 4:

- Option 1: on - Implies placing something onto a surface; not the best fit here.

- Option 2: down - The correct choice. "Write down" is a common phrase meaning to record something, like jotting tasks on a to-do list.

- Option 3: of - Doesn't connect logically in this context.

- Option 4: onto - Implies movement onto a surface, not suitable here.

Given these points, the best choice is "write down your to-do list."

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