I thought that if I take some information from a paper and use it in my paper, then only that paper should be cited. Can someone please help me to clarify it?
If you take information directly from another paper, that is one circumstance where you should obviously cite it. That is a sufficient but not necessary condition, and it doesn't really answer the broader question: what papers should I mention in my paper? Generally speaking, you should cite papers that are sufficiently closely related to the topic/point at hand that they will be helpful to the reader. This is a contextual decision, and it is usually informed by choices relating to the goal of your own paper, the consequent scope and depth of your literature review, etc.
Most papers will begin with an introduction that "sets the scene" for what you are doing, describes the problem at hand, and discusses literature related to the problem. Sometimes you will do a systematic literature review, but even if this is not done, you will usually want to give some context that mentions other important papers relating to the problem, method, etc., that you are using. The reason to do this is that it is helpful to the reader to learn what has already been done in the field and it provides them with other sources where they can learn more about the problem, methods, etc. Papers vary substantially in the amount of detail they give on other literature; some papers give only narrow citations of major works while others give a broad literature review.
In your case, the four papers suggested by the reviewer are related to your paper (you do not specify how) so the reviewer is suggesting that you should cite them in your paper at an appropriate point. Since you have identified 15-20 papers of this kind, you will need to make a decision on whether it is useful to cite them all, or just the major ones, or some selection of your own choosing. You should be guided by putting yourself in the reader's shoes --- what other works would it be helpful for them to know about while reading your paper?
Since you are a PhD student, you should seek guidance from your supervisor on specifics and run your ideas by your supervisor to ensure that what you are doing is good practice in your field. The above gives some general advice, but your supervisor will be able to give more specific scrutiny to this particular case.
Is it necessary to also cite those papers which have been published when my paper was in review?
Firstly, it is not necessary to cite any paper in your field simply because it exists. You should always be guided by the goal of your own paper, the desired scope of discussion of other literature, the importance and relevance of other papers, etc. For papers that have only recently been published there is the added problem that they may not have existed when you submitted your paper, or you might not yet have formed a view on the relevance/importance of new work.
If your paper is already in review then you cannot change it right now, but you might get an opportunity to change its substance one you get the referee reports back (e.g., under a "revise and resubmit" scenario). Assuming you have an opportunity for revisions (beyond merely proof corrections) you can use this opportunity to add citations to new papers if you wish. Again, there is no necessity to add these new papers just because they exist, and you should be guided by what is helpful for the reader.
