So,i think that the fourth theme is pretty good for you to talk about.Cause there are so many subjects to talk.You can discuss it from these aspects as follow:
you can say something about your interests.like singing,dancing,palying balls and so on.
you can say someting about your extracurricular time that how can you  utilize.
you can say someting about every thing in this subject casue your spare time possibly you can do everything what do you want to do.
So,you kown,i think this theme is perfect for you.