You can add music in your best four only when you are doing mains in music i.e. BA in music, Honours in music etc. If you add music otherwise, then some part of your percentage will be deducted. For example, if you choose English, Music and two other subjects from academic subject list and your percentage is coming out to be 80, but it will be considered 78-79% after the deduction for adding music in best four subjects.
Please specify the university and course you want admission in, to know about the criteria in more detail.
I hope this information helps you.
Depending on the course you are taking admission, you can take the subjects. For example, if you are taking admission in History Hons course then you must have History and English subject compulsory and apart from this you can take another two subjects for the best four. So you have to choose it accordingly and also it sometimes on the college you take admission in. If you are choosing subjects such as Fine Arts, Music etc then you can undoubtedly take it. Also it doesn't matter which subject you choose, the best of four subject is the subject in which you have the best results. So if Music is one of the best scored subjects then you can take it. But please do keep in mind that you should keep the compulsory subject as required by the college in your best four and along with that you can take music as one subject.
I hope this answer helps. All the very best for your future endeavors!