Is it okay to ask prospective supervisor to recommend some article to read before joining his group for PhD? A few days ago, I received my offer letter to join the Ph.D. program. I was about to inform my prospective supervisor (via e-mail) that I have received this letter from the graduate office. I will be joining the program in August 2019.

I don't want to waste my 4 months. Is it okay if I ask my supervisor to recommend me some articles/topics to read in these four months?
Yes, definitely ask for some recommended reading! In fact, I wouldn't be surprised if your new supervisor suggests some papers to you even without prompting (mine did). As mentioned in the comments, supervisors are generally pleased when their students show an interest in the topic and want to learn more. You can look at some recent papers by your supervisor to get an idea of where to start, too.