What is a good topic for a "4-page term paper" on Manifest Destiny?
In general, a topic is “good” if it fulfills two criteria. First, it has to be broad enough for you to fill the required number of pages in answering it. Second, it has to be something that is of interest to you. We can help you with the first criterion, but not with the second.
One thing you could do would be to write about the nature of manifest destiny. For example, you could ask why the idea of manifest destiny arose in the United States. Alternately, you could ask if manifest destiny was primarily a racist idea. For both of those topics, you would need to lay out all of the ideas of manifest destiny. You would then need to try to figure out why Americans would feel that way (for the first question) or whether racism was the most important of these ideas (for the second question).
You could also look at the impact of manifest destiny. You could try to determine who it hurt and who it helped. Relatedly, you could ask if/how manifest destiny led to any other events in our history. Did it help to cause the Civil War? Did it help to bring about the industrialization of the second half of the 19th century?
Any of these questions that is of interest to you could make a good topic for this paper.