I basically read this just to get my fix while waiting for Insurgent from the library. It's written in the same familiar writing style, we all feel in love with in Divergent. It replays the knife throwing scene, but from Four's POV. 
 Yes, it shows us how limited Tris' point of view is, and how blind it makes her to Four's feelings for her. The problem is that it's difficult, in such a short scene, for Roth to make Four's inner voice distinctive. I didn't get any amazing insights from reading this, and feel that it was unnecessary. Fun, yes, but unnecessary.