It only took me an afternoon to jam out all four of these short novellas. They are all less than 100 pages and take on small parts of Four's journey in Dauntless. The first three are before Divergent and the The Traitor is during Divergent. They are all three to four star ratings and worth the read, but I would definitely take them on after you've read Divergent. They wouldn't make sense otherwise.
You should also try Free Four. Its Four's side to the knife throwing scene.
There's also - The Path to Allegient #2.5 and We can be Mended #3.5, but I haven't heard anything good about these two, so I have no desire to read them. If I happen to get my hands on a copy by chance, I will let you know if it was worth it.
There are so many books in this Dystopian world that has captured the minds of the world. Pick up a book by Veronica Roth and share your feelings on how you liked them. I would love to hear your take.