Explanation:

- Option 1: "as" - This would be used to indicate that something is being done in the same manner or for the same reason as something else.

- Option 2: "than" - This is used for comparisons, and doesn't fit well in this sentence structure since it doesn't compare two elements.

- Option 3: "then" - Indicates a sense of time, suggesting the next step or consequence, but this doesn't fit the structure correctly.

- Option 4: "so" - This indicates causation or a result and fits well in the sentence, suggesting the result of having no other option.

Correct Answer: Option 4: "so"