I'm an undergraduate senior and will soon be applying for competitive PhD programs in computer science in the US/Canada. I already have three CS professors whom I have good relationships with (as a student, TA, and RA) as my recommenders. However, I also have a music professor whom I worked closely with on a purely music research in my first two years in college. The paper from that research has been accepted to a journal but not published yet.
Some universities allow up to five recommendation letters. So, should I ask my music professor to be my fourth recommender? If so, what should I ask them to highlight in the letter?