Your question suggests that this is something that has been covered in a textbook or in a lecture and that you are expected to produce four reasons in particular, those covered in your course. However, as a general matter, you are likely to get different answers from different people. I can offer five reasons that I consider the most important reasons we study literature, which are that it allows us to move out of ourselves, that it allows us to look within ourselves, that it makes us empathetic, that it gives us common ground with other educated people, and that it is pleasurable in and of itself.
You will hear English teachers tell you that literature is a window upon the world, and this is so true. Literature allows us to visit different times, different places, and different peoples. Without it, all we would know about is a very limited piece of life. When I read The Kite Runner, I learn about Afghani culture, the trials and tribulations the country has experienced, and what it is like to be an immigrant. When I read Reading Lolita in Tehran, I learn about still another culture, and I learn about how women and intellectualism are repressed in some other countries. When I read Huckleberry Finn, I am able to go back over a hundred years in the history of my own country, to learn about what life was like then in times of slavery and before this country was tamed. Who would want to deny themselves these wonderful windows onto the world out there?
You may also hear that reading literature is like a mirror. What is meant by this is that it allows you to look at yourself. As we read, we notice how characters may be like us or not. We notice how situations might be similar to those we have experienced. This has the effect of making us examine ourselves critically. What would we do in such a situation? Would we act as particular character would act? Sometimes, in my book group, this is the sort of discussion we have, and different people have different opinions about what is right or wrong, but the book we are discussing seems to make us focus upon ourselves, a way of asking if we measure up, if we are of good or bad character. Enders Game seems to be a mirror for readers, who think about what their own moral characters are. Similarly, The Giver tends to make people examine themselves, to ask if they are really thinking for themselves or allowing others to force them into conformity. So, as we read, we are holding up a mirror to examine ourselves.
Having these windows and mirrors makes us more empathetic people. We are now able to have some insight into how other people live, how other people feel, how circumstances force people to make difficult choices that are not perfect. We are able to understand how someone can do everything right and still have a poor outcome because of circumstances beyond control. We can look deeply within ourselves and ask whether or not we are judgmental of others in a a way that is not very attractive. All of this encourages us to understand the feelings and motivations of those around us, which is what empathy really is. There have been studies done that have concluded that those who read literature are more empathetic than those who do not. And in order to get along in a diverse world, the most useful kind of emotional intelligence is probably empathy.
As you move along in the world, you will find that many educated people have read the same literature. This gives you a common ground upon which to communicate. Often when I meet strangers, we end up talking about the books we have read, and it is remarkable how many of the same literary works we have in common. This gives us a common frame of reference, a common vocabulary, and common literary allusions. Literature is the stuff of civilization, and when we read it, we become part of a tribe of civilized people. That is a membership I cherish.
Last, but not least, reading literature is one of life's great pleasures. It is as pleasurable as a good meal or a walk in the park on a beautiful day. When I deserve a reward after a great deal of hard work, I often reward myself with reading. A book is a wonderful companion, not to replace people, but still a wonderful experience in and of itself. Even if all the previous reasons did not exist, I would still be a reader, just for the pleasure of it!