This book made me understand Tobias so much more than I did before. I decided to read it quite a long time after I finished Allegiant, so I would have time to think about the trilogy and the characters. It was a good choice, and even months later, I felt as if I had just finished the trilogy.
It was hard to read about Tobias's feelings towards his father, because no one should experience such things. I felt insanely sad for him and desperate to help him, even more than I did while reading the trilogy. I can now say that I love Tobias with all my heart, because I now know every motive behind his actions and completely understand them.
The only thing that I disliked was the kind of summary of everything that happened at the beginning of each part, because it really felt repetitive, since I had just read about everything. I know it's because every part is a different story, but I would have liked not to have it in a collection of those stories. I pretty much forgot about it after reading another page, so it was fine.
Overall, I think that everyone who has read the Divergent trilogy should read Four, because I felt like this was a part of the trilogy as well as the other books, not just an extra. I loved it!