The Circle Opens quartet by Tamora Pierce

This is the second series that focuses on the characters of Sandry, Tris, Daja, and Briar. (I talk about the first series, the Circle of Magic, here.) In this second series, the four teenagers are older, fourteen years old or thereabouts, and they are all separate from each other. Each of them, except for Sandry, are traveling with their teachers, and are too far away from each other to maintain their magical connection. This makes it so that the stories are much more focused on each of the characters individually, rather than on the group as a whole.
The first book, Magic Steps, focuses on Sandry, who has stayed at home to care for her uncle. The story begins with Sandry discovering a young mage, and realizing that it is her duty to train him. It also begins with a gruesome murder, and so the story follows both her training and the tracking of the murderers. And of course, it takes her new pupil to help her catch the perpetrator. Each of the other three books follows this basic pattern. Each young mage comes across someone who they discover and must train, at the same time as they are combating the nefarious deeds of the bad guys. And of course they are learning about themselves as individuals, and are growing up quite a bit in the process.
This series was entertaining, although the murder aspect of it did get to be a bit much. Just a little bit too coincidental, for obvious reasons. But it did allow for more character development, which was interesting, and created a good base to build further series on. The other books in this series are Street Magic(Briar), Cold Fire (Daja), and Shatterglass (Tris).