Imagine in a conversation over an important issue, between two people, the other side starts talking about a different topic which differs with the main subject under discussion. This way in an unwanted manner he is misleading the conversation. In other hand you intend to come back to the subject you started to talk to him about it at the beginning. Which one of the following sentences sounds natural in (especially American) English?
1 - Let's get back to the main subject.
2 - Let's get back to the main matter.
3 - Let's get back to the main topic.
4 - Let's get back on topic.
If no one, then please tell me the most common way to say that.
Added: to clarify the matter in my question I will make two different scenarios:
Father: You have put much more time for your studies. You don't study as hard as you should recently David. I was shocked when I saw your last report card at the end of the previous semester just a couple of days ago.
Son: I will do that dad. Meanwhile, did you know that tomorrow will be a derby between Manchester City and Manchester United. (Smilingly says the son.)
Father: Yes. I'm informed about it. But......[the phrase]........
Professor: Today we are going to discuss about the latest discoveries in the field of oncology. But before that I have some small grievances against some students that do not pay attention to me when I teach. Last week, at the end of a session, a student came to me and asked me about a subject that we had discussed about it at least 30 minutes during the last three sessions. ....[The professor asks them to pay more attention to the course topics.] Well. ......[the phrase]........