Explanation:

- The context of the passage is about boys selected by Lord Baden Powell who worked as messengers.

- For blank No. 4, we are looking for a word that describes the task or duty performed by the boys.

- Option 1: post – This term refers to a position or assigned duty, but it doesn’t fit the context of task performance.

- Option 2: thing – This is too vague and doesn't fit appropriately as a description of a task.

- Option 3: job – This fits well as it refers to a task or duty that they performed successfully.

- Option 4: work – While this could be suitable, "job" is more specific and descriptive within the context. ??

- The most appropriate option is option 3: job.