Apparently, in the future, everyone gets to play with knives (and probably run with scissors). So it really is a utopia.
Today's Dauntless lesson, led by Eric the Crazy, is in how to throw a knife. And Tris is pretty good at it.
And—surprise, surprise—Al isn't very good at this at all. In fact, Al is so bad at the crucial skill of knife throwing, that Eric wants to teach Al a lesson about bravery or something by using Al as a target. (Which reminds us of this, not-for-the-squeamish scene.)
But Tris steps in: Four can throw knives at her instead of Al.
So, Four throws the knives, narrowly missing Tris. And Tris thinks he's trying to make her fail by teasing her.