Complex sentence: On her return to Dauntless headquarters Tris is attacked due to her success in training, with Peter, Al, and Drew attempt to throw her into the chasm, but she is rescued by Four.
Split into simpler sentences: On her return to Dauntless headquarters Tris is attacked by Peter, Al and Drew due to her success in training. The three initiates attempt to throw her into the chasm, but she is rescued by Four.