【答案解析】It is inappropriate to assert that the Chinese particle expresses the completion of an action. The particle le does sometimes have such a meaning, but that is all because of its function as a marker of the changing of a certain condition.
The particle in these four sentences shares such a common semantic feature that it illustrates the changing, either of time, of action, or of condition. In the first sentence, the particle helps transfer the information that the reader the weather is getting cold, which means it was hot or not so cold in the previous time. In the second sentence, with the combination of yao, the particle expresses an action which is about to happen in the near future, and shows the change of the weather, or of the condition. As to the third sentence, although it is clear that at the moment of speaking the speaker is actually not away, yet the particle conveys a tendency of the change of an action. Lastly, in the fourth sentence, the particle le also implicates the gradual process from lightness to darkness, which is also the evidence for its role as a condition-changing marker.